Chapter 8.392 — ACCESSORY DWELLING UNITS

Accessory dwelling units are a residential use that provide an important source of housing. The purpose of this Chapter is to:

1. Increase the supply and diversity of the County’s housing stock, in particular the number of smaller and more affordable units, by allowing accessory dwelling units to be built on existing residential properties, while preserving the neighborhood character.

2. Increase the housing stock of existing neighborhoods in a manner that has less impact on the environment than development of housing in undeveloped areas.

3. Allow more efficient use of existing residential areas and supporting infrastructure.

4. Provide a means for residents to remain in their homes and neighborhoods.

5. Provide opportunities for homeowners to earn supplemental income from renting an accessory dwelling unit.

6. Establish standards for accessory dwelling units to ensure that they are safe, habitable, and compatible with existing development.

1. Accessory Dwelling Unit. An “accessory dwelling unit” is a dwelling unit located or proposed to be located on a lot which contains, or will contain, a primary residence. Accessory dwelling units may be detached from or attached to the primary residence on the property. Accessory dwelling units may also be (1) efficiency units, as defined in Section 17958.1 of the California Health and Safety Code, or (2) manufactured homes, as defined in Section 18007 of the California Health and Safety Code. Accessory dwelling units are “accessory dwelling units” as that term is used in Government Code Section 65852.2. A “second unit” or “secondary unit” is an accessory dwelling unit. Accessory dwelling units are not “accessory buildings” as defined in Section 8.04.030(22). Any secondary structure that provides independent facilities for living; sleeping; eating; cooking; and sanitation, may be considered an accessory dwelling unit, at the discretion of the Director of Planning and Building, unless an applicant can provide compelling evidence to the contrary to the satisfaction of the Director of Planning and Building.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

2. Attached Accessory Dwelling Unit. An “attached accessory dwelling unit” is a unit that is built as an addition to, extension of, or within the primary residence.

3. Detached Accessory Dwelling Unit. A “detached accessory dwelling unit” is a unit that is an independent structure, entirely separated from the structure of the primary residence. Accessory dwelling units constructed within, or as an extension of an existing detached structure other than the primary residence are considered detached accessory dwelling units.

4. Efficiency Kitchen. An efficiency kitchen, as defined in Government Code Section 65852.22, is a kitchen that contains at least a cooking facility with appliances, and a food preparation counter and storage cabinets that are of reasonable size in relation to the size of the junior accessory dwelling unit.

5. Floor Area. For purposes of this Chapter, the “floor area” of an accessory dwelling unit is the area of each floor level included within the walls enclosing each dwelling unit. The floor area shall be measured from the outside face of the walls enclosing each dwelling unit including all closet space and storage areas contained within the unit, including habitable basements and attics, but not including unenclosed porches, balconies, or any enclosed garages or carports. For purposes of calculating allowable floor area of accessory dwelling units based on a proportion of the size of the primary residence, only the livable floor area of the primary residence shall be counted. The floor area of any other structures, for purposes of calculating total floor area, lot coverage, or other calculations, shall be calculated in the manner described in the relevant zoning regulations.

of calculating allowable floor area of accessory dwelling units based on a proportion of the size of the primary residence, only the livable floor area of the primary residence shall be counted. The floor area of any other structures, for purposes of calculating total floor area, lot coverage, or other calculations, shall be calculated in the manner described in the relevant zoning regulations.

6. Junior Accessory Dwelling Unit. A “junior accessory dwelling unit” is an accessory dwelling unit built entirely within the walls of an existing or proposed primary residence, not exceeding five hundred (500) square feet in size, and including an efficiency kitchen, as described in Government Code Section 65852.22. A junior accessory dwelling unit may include separate sanitation facilities or may share sanitation facilities with the existing or proposed primary residence.

7. Multifamily Dwelling. Two (2) or more attached dwellings on a single lot is considered a multifamily dwelling. Multiple detached single-unit dwellings on the same lot are not considered a multifamily dwelling.

8. Owner Occupancy. Owner occupancy is the condition and requirement that the owner of a parcel on which a junior accessory dwelling unit is constructed live in one (1) of the units on the property in perpetuity.

9. Primary Residence. A “primary residence” is the main residence, including attached habitable and uninhabitable spaces, located or proposed to be located on the parcel on which an accessory dwelling unit(s) is located or proposed to be located.

10. Stepback. A setback above the ground floor, where the building is “stepped back” an additional distance from the applicable property line.

11. Uninhabitable space. Uninhabitable space is space that is not developed to full standards of habitability.

Accessory dwelling units shall be allowed in the R-1, R-2, R-3, PC, CMU-1, CMU-2, CMU-3, NMU, NMU-ECR, R-E, RH, RM and TPZ districts, and all other districts in which residential uses are permitted, regardless of any regulations that might otherwise prohibit accessory dwelling units in those districts.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

Accessory dwelling units meeting all of the applicable requirements of Sections 8.392.050 through 8.392.090, shall be approved ministerially, without public notice, public hearing, or discretionary review.

Accessory dwelling units not meeting the applicable standards set forth in Section 8.392.050 through 8.392.090 will be considered a conditionally permitted use within the districts specified in Section 8.392.030 and may be permitted by a conditional use permit pursuant to a public hearing before the Zoning Hearing Officer, as described in Section 8.392.110.

New accessory dwelling units shall be subject to the requirements and standards described in this Chapter. Where not superseded by the specific requirements described in this Chapter, accessory dwelling units shall also be subject to the requirements applicable to any dwelling unit on the same parcel in the same district. Development standards applicable to all accessory dwelling units include the following:

1. Minimum Lot Area. Accessory dwelling units shall be exempt from the minimum lot area per dwelling unit provisions in the applicable district.

2. Minimum Lot Size. Accessory dwelling units shall be exempt from all minimum lot size requirements.

3. Maximum Density of Development. Accessory dwelling units shall be exempt from any and all provisions limiting the maximum density of development in the applicable district.

4. Setbacks. Notwithstanding the required setbacks in the applicable district, minimum setbacks for accessory dwelling units shall be:

   • a. Front Setbacks. With the exception of accessory dwelling units created entirely within the space of an existing structure, for all other accessory dwelling units regardless of height, the accessory dwelling unit may be located no closer to the front property line of the subject parcel than the lesser of:

     • (1) The front setback required by the relevant zoning district, or

     • (2) The distance from the front property line of the primary residence located or proposed to be located on that parcel. For purposes of this section, the primary residence includes attached garages. In cases where the application of the setbacks described in (1) and (2) would preclude the creation of an accessory dwelling unit of at was eight hundred (800) square feet with side and rear setbacks of at least four (4) feet, the accessory dwelling unit may be located within the front setback, but must be located as far as possible from the front property line.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

In cases where an existing primary residence is closer to the front property line than the front setback normally required in the applicable district, the accessory dwelling unit shall also be allowed to be located as close to the front property line as the primary residence.

In cases where an accessory dwelling units are proposed to be built atop, or below an existing detached garage that is located within the required front setback, a conditional use permit shall be required unless this provision would preclude the creation of an accessory dwelling unit of at least eight hundred (800) square feet.

• b. Side and Rear Setbacks. For accessory dwelling units created entirely within the space of an existing structure, or constructed in the same location and to the same dimensions as an existing structure, side and rear setbacks shall be those already existing for that structure.

For all other accessory dwelling units regardless of height, the accessory dwelling unit may be located no closer to the property line than: Side: four (4) feet Rear: four (4) feet.

• c. Stepbacks. Accessory dwelling units, whether attached or detached, exceeding sixteen (16) feet in height shall have the following stepbacks from the applicable property lines, located at a point no higher than sixteen (16) feet on the structure: Side: five (5) feet Rear: ten (10) feet.

5. Floor Area. The allowable floor area of an accessory dwelling unit shall be calculated in the manner described in Sections 8.392.050 through 8.392.090, but in no case shall these regulations be applied in such a way as to preclude an attached or detached accessory dwelling unit of up to eight hundred (800) square feet in size that meets all other relevant standards.

6. Lot Coverage. Accessory dwelling units shall count against the allowed lot coverage on a parcel, except that no lot coverage restriction shall preclude creation of an attached or detached accessory dwelling unit of up to eight hundred (800) square feet in size that meets all other relevant standards.

7. Height. The maximum height of an accessory dwelling unit shall be twenty-six (26) feet. Building height shall be measured as the vertical distance from any point on the lower of (a) finished grade, or (b) natural grade, to the topmost point of the building immediately above. Chimneys, pipes, mechanical equipment, antennae, and other similar structures may extend up to eight (8) feet beyond the building height, as required for safety or efficient operation. Accessory dwelling units built entirely within an existing building shall be subject to the greater of the height limit applicable to that building in the relevant district, or the maximum height of the existing primary residence, measured in the manner described in the Zoning Regulations of the relevant district.

8. Daylight Plane. Neither accessory dwelling units built above an existing detached or attached garage or accessory structure, nor detached accessory dwelling units taller than sixteen (16) feet in height, shall be subject to daylight plane requirements. Daylight plane applies to accessory dwelling units attached to or above existing single-family residences and taller than nineteen (19) feet in height, except daylight plane shall be measured from the setback applicable to the accessory dwelling unit.

9. Balconies and Decks. Accessory dwelling units that do not meet the setback requirements that would apply to a primary residence in the same district shall have no rooftop decks, and no portion of any balcony or deck shall be located above ten (10) feet in height, exclusive of railings, measured in the same manner as

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

height in Section 8.382.050(7), except on the side of the accessory dwelling unit facing the primary residence. Accessory dwelling units that meet the setback requirements that would apply to a primary residence in the same district may have rooftop decks and balconies to the extent otherwise allowed in the relevant district.

10. Windows. Accessory dwelling units that do not meet the setback requirements that would apply to a primary residence in the same district shall have no windows located above or extending above ten (10) feet on the accessory dwelling unit, measured from finished grade, except on: (1) the side(s) of the accessory dwelling unit facing the primary residence, and (2) the side(s) of the accessory dwelling unit that comply with the setback requirements of the district. On the sides of the accessory dwelling unit that do not meet the setback requirements of the district, clerestory windows located above ten (10) feet on the accessory dwelling unit shall be allowed, if they have a lower sill height of no less than six (6) feet from the nearest interior floor of the accessory dwelling unit, and a total window height no greater than twenty-four (24) inches. Skylights shall be allowed.

11. Fire Safety Requirements. For accessory dwelling units that do not meet the setback requirements of Section 8.392.050(4)(b) are required to provide fire-rated walls, windows, eaves, and other architectural features on the side(s) of the accessory dwelling unit that encroach on the setback. No egress windows shall be allowed on the side(s) of accessory dwelling unit that do not meet the setback requirements of Section 8.392.050(4)(b)

12. Ingress and Egress. Accessory dwelling units shall have an independently accessible entrance that does not require passage through the primary residence.

13. Required Facilities. With the exception of junior accessory dwelling units, which are subject to the requirements described in Section 8.392.080, all attached or detached accessory dwelling units must include the following:

• a. Independent facilities for living, sleeping, eating, cooking, and sanitation.

• b. A kitchen area containing a refrigerator, sink, and permanently installed cooking appliance, which must include at least a fixed stovetop.

• c. A fully plumbed bathroom including sink, shower, and toilet.

14. Parking Requirements

• a. Required Parking. One (1) new covered or uncovered parking space, in addition to those already existing on the parcel, shall be provided on-site for each new attached or detached accessory dwelling unit.

• b. Parking Exemptions. Accessory dwelling units meeting any of the following criteria shall not be required to provide any parking in addition to that already provided on the parcel, or in the case of a concurrent application for a new primary and accessory dwelling unit, shall not be required to provide any parking in addition to the parking required for the primary residence:

  • (1) Accessory dwelling units located within one-half (1/2) mile walking distance of a public transit stop or station. Public transit stops must be served by a transit line serving the public, and not solely by specialized, private, or limited population services such as school buses, privately run shuttles, or other services that cannot be used by all public riders.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

• (2) Accessory dwelling units located within a designated architecturally and historically significant historic district.

  • (3) Accessory dwelling units that are part of the existing primary residence or an existing accessory structure, including attached or detached garages.

  • (4) Accessory dwelling units located within one (1) block of a car share vehicle pick-up/drop-off location.

  • (5) No parking exemption can be granted for properties (1) located within Wildland Urban Interface areas, which includes: Emerald Lake Hills, Devonshire, and other areas designated by the Fire Department as Wildland Urban Interface areas, and (2) where fire engine access and turnaround space required by California Fire Code: Fire Appendix D is not provided.

• c. Conversion of Covered Parking. Any covered parking removed in order to create an accessory dwelling unit is not required to be replaced, and if voluntarily replaced, may be replaced with uncovered parking of any type and configuration allowed by Section 8.392.050(14)(f), below. For purposes of this Section, conversion includes partial or full demolition of covered parking required to create an accessory dwelling unit.

• d. Garage Conversion. If an existing attached or detached garage is converted to an accessory dwelling unit, the parking previously provided by that garage is not required to be replaced, and if voluntarily replaced, may be replaced by uncovered parking of any type and configuration allowed by Section 8.392.050(14)(f), below, and no additional parking related to the accessory dwelling unit is required. For purposes of this Section, conversion includes partial or full demolition of the garage and partial or full replacement with an accessory dwelling unit.

• e. Use of Existing Parking. If the parking already existing on the parcel exceeds that required for existing development on the parcel, excess parking spaces shall be counted toward the new parking required for the accessory dwelling unit.

• f. Provision and Location of Parking. Parking spaces shall be provided in the following manner:

  • (1) Pervious Surfaces. All new uncovered parking spaces created for the accessory dwelling unit must be provided on pervious surfaces. The maximum amount of impervious surfaces designated to satisfy the accessory dwelling unit parking requirement shall be no greater than the amount of impervious surfaces existing at time of application. Existing impervious surface area may be used for parking and need not be converted to pervious surface.

  • (2) Uncovered Parking. All parking required for the accessory dwelling unit may be uncovered.

  • (3) Front or Side Yard Parking. Up to three (3) parking spaces may be provided in the front or side yard setbacks. Not more than six hundred (600) square feet of the front yard area shall be used for parking.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

• (4) Tandem Parking. Required parking spaces for the primary residence and the accessory dwelling unit may be provided in tandem on a driveway. A tandem parking arrangement consists of one (1) car behind the other. No more than three (3) total cars in tandem may be counted toward meeting the parking requirement.

  • (5) Compact Spaces. All parking required for the accessory dwelling unit may be provided by compact parking spaces, as defined in Section 8.344.020(a).

15. Requests for Parking Exceptions. If the required parking for an accessory dwelling unit cannot be met in accordance with this Chapter, an application may be submitted for a parking exception, as specified in Section 8.344.040. For parking provided in accordance with the provisions of this Chapter, a parking exception shall not be required.

16. Design Review. Accessory dwelling units shall not be subject to design review. Compatibility with applicable design standards for such units shall be determined by staff, at the discretion of the Director of Planning and Building.

17. Architectural Review. Accessory dwelling units located in scenic corridors are not subject to architectural review.

18. Concurrent Application for Development of Primary Residence and Accessory Dwelling Unit. In the case of a concurrent application for development of a new primary residence and new accessory dwelling unit on the same parcel, whichever unit is first issued a certificate of occupancy must conform to all applicable regulations for the primary residence in the relevant district.

19. Conversion of Existing Residence. An existing residence may be converted to an accessory dwelling unit in conjunction with development of a new primary residence, if the existing residence, once converted, will meet all the standards applicable to development of a new accessory dwelling unit described in this Chapter.

20. Conversion of Accessory Buildings. An accessory dwelling unit may be constructed within, above, or below an existing, detached accessory building, provided the resulting unit conforms to all applicable provisions of this Chapter.

Accessory dwelling units constructed within, above, or below an existing, detached accessory building that conforms to all applicable provisions of this Chapter shall not be required to obtain a use permit, regardless of the requirements of the applicable district.

Accessory dwelling units built within, above, or below existing, or new garages are subject to the specific provisions of this Chapter regarding such units, regardless of any regulations to the contrary elsewhere in the Zoning Regulations.

21. Creation of Accessory Dwelling Unit Entirely Within a Non-Conforming Primary Residence.

In the case of an existing primary residence that does not conform to one (1) or more provisions of the zoning regulations, creation of an accessory dwelling unit that will be entirely within the existing primary residence shall not, in itself, create a requirement that the nonconformities be rectified. No correction of existing nonconformities shall be required, unless they pose a threat to public health and safety.

sting primary residence that does not conform to one (1) or more provisions of the zoning regulations, creation of an accessory dwelling unit that will be entirely within the existing primary residence shall not, in itself, create a requirement that the nonconformities be rectified. No correction of existing nonconformities shall be required, unless they pose a threat to public health and safety.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

22. Short Term Rental. Accessory dwelling units created pursuant to the provisions of this Chapter, if rented, shall only be rented for a term longer than thirty (30) days.

23. Impact Fees. Accessory dwelling units of less than seven hundred fifty (750) square feet in size shall be exempt from all impact fees. Accessory dwelling units of greater than seven hundred fifty (750) square feet in size shall only be charged impact fees in an amount equal to the standard impact fee for such a unit, multiplied by the proportion of the square footage of the accessory dwelling unit to the square footage of the primary dwelling unit.

Detached accessory dwelling units shall be subject to the requirements described in Section 8.392.050, and to the following requirements:

1. Distance Between Detached Accessory Dwelling Units and Other Residential Structures. The distance required between a detached accessory dwelling unit and any other residential structure on the same parcel must be a minimum of five (5) feet, measured from foundation to foundation. If a separation distance greater than five (5) feet is required by any other section of the Zoning Regulations, it shall be disregarded, and the standards of this Chapter shall govern.

2. Floor Area of Detached Accessory Dwelling Units. Notwithstanding any floor area standards applicable to accessory dwelling units in the applicable district, the following floor area standards shall apply:

   • a. The floor area of a detached accessory dwelling unit shall not exceed eight hundred (800) square feet or thirty-five percent (35%) of the livable floor area of the existing or proposed primary residence, whichever is larger, up to a maximum of one thousand five hundred (1,500) square feet. The floor area of the primary residence shall be calculated in the manner described in the relevant base or overlay district Zoning Regulations.

   • b. The floor area of a detached accessory dwelling unit shall count against the total floor area allowed on a parcel, such that the total floor area of the accessory dwelling unit in combination with the square footage of the primary residence and other structures on or proposed to be on the parcel shall not exceed the maximum floor area allowed within the zoning district, with the following exception:

     • (1) Regardless of floor area limitations, a single eight hundred (800) square foot detached accessory dwelling unit shall be allowed on a parcel, so long as that accessory dwelling unit can meet the setback and stepback requirements described in Section 8.392.050.

3. Detached Accessory Dwelling Units and Junior Accessory Dwelling Units. One (1) detached accessory dwelling unit may be built in combination with one (1) junior accessory dwelling unit on the same parcel, as long as both units comply with all relevant provisions of this Chapter. A detached accessory dwelling unit may not be built in combination with an attached accessory dwelling unit on the same parcel.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

Attached accessory dwelling units shall be subject to the requirements described in section 8.392.050, and to the following requirements:

1. Floor Area of Attached Accessory Dwelling Units. Notwithstanding any floor area standards applicable to accessory dwelling units in the applicable district, the following floor area standards shall apply:

   • a. The floor area of an attached accessory dwelling unit shall not exceed eight hundred (800) square feet or fifty percent (50%) of the livable floor area of the existing or proposed primary residence, whichever is larger, up to a maximum of one thousand five hundred (1,500) square feet. The floor area of the primary residence shall be calculated in the manner described in the relevant base or overlay district Zoning Regulations.

   • b. The floor area of an attached accessory dwelling unit shall count against the total floor area allowed on a parcel, such that the total floor area of the accessory dwelling unit in combination with the square footage of the primary residence and other structures on or proposed to be on the parcel shall not exceed the maximum floor area allowed within the zoning district, with the following exception:

     • (1) Regardless of floor area limitations, a single eight hundred (800) square foot attached accessory dwelling unit shall be allowed on a parcel, so long as that accessory dwelling unit can meet the setback and stepback requirements in Section 8.392.050.

     • (2) For attached accessory dwelling units built entirely within the walls of an existing or proposed primary residence, an additional one-hundred fifty (150) square feet of floor area is allowed regardless of other floor area limitation, solely for the purpose of providing ingress/egress, and not for expanded living space. Such space for ingress and egress typically includes, but is not limited to, stairs, porches, foyers, and other similar areas.

2. Ingress and Egress for Attached Accessory Dwelling Units. With the exception of junior accessory dwelling units, attached accessory dwelling units shall only be allowed a connecting doorway or other permanent ingress or egress between the primary residence and the accessory dwelling unit with the approval of the Director of Planning and Building, at the Director's discretion. In all cases, such doorways must be independently securable from within the accessory dwelling unit and from within the primary residence.

For accessory dwelling units attached to the primary residence, any new entrances and exits may face the front of the parcel only if they are 1) located so as not to be visible from the front of the parcel, 2) unless otherwise, required by clearance and or landing requirements, or 3) permitted by the Director of Planning and Building, at the Director’s discretion.

3. Attached Accessory Dwelling Units and Junior Accessory Dwelling Units. One (1) attached accessory dwelling unit may be built in combination with one (1) junior accessory dwelling unit built on the same parcel, as long as both units comply with all relevant provisions of this Chapter. A detached accessory dwelling unit cannot be built on the same parcel as an attached accessory dwelling unit.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

Junior accessory dwelling units shall be subject to the requirements described in 8.392.050, with the following exceptions:

1. Location. Junior accessory dwelling units must be constructed entirely within the walls of an existing or proposed primary single-family residence, including within the walls of a garage attached to the primary residence, except that an additional one hundred fifty (150) square feet may be built solely for the purpose of providing ingress and egress for the junior accessory dwelling unit.

2. Floor area. The floor area of a junior accessory dwelling unit may be no greater than five hundred (500) square feet under any circumstance, except that an additional one hundred fifty (150) square feet may be created outside of the primary residence, solely for the purpose of providing ingress and egress for the junior accessory dwelling unit.

3. Required Facilities. Junior accessory dwelling units must have a sleeping area, sink, and efficiency kitchen as defined in Government Code Section 65852.22. Junior accessory dwelling units may share a bathroom with the primary residence.

4. Internal Ingress and Egress. Junior accessory dwelling units must have external ingress and egress, as described in Section 8.392.050.12. However, junior accessory dwelling units may have an internally connecting doorways between the junior accessory dwelling unit and the primary residence. The internally connecting doorway must be independently securable from both the junior accessory dwelling units and the primary residence. An internal connecting doorway between the junior accessory dwelling unit and the primary residence is required if the junior accessory dwelling unit does not include a separate bathroom.

5. Owner Occupancy. The owner(s) of the parcel on which a junior accessory dwelling unit is proposed shall be required to occupy one (1) of the units on the parcel. The owner(s) shall be required to record a deed restriction enforcing this requirement, which shall run with the land, and which shall be provided to the Planning and Building Department. The deed restriction shall also include a prohibition on the sale of the junior accessory dwelling unit separate from the sale of the single-family residence, including a statement that the deed restriction may be enforced against future purchasers.

6. Parking. No additional parking shall be required for the creatin of a junior accessory dwelling unit.

On parcels with existing or proposed multi-family structures, including multi-family structures with two (2) or more units, multiple accessory dwelling units shall be allowed, subject to the requirements described in Section 8.392.050, and to the following requirements:

1. Accessory Dwelling Units within Multi-family Structures:

   • a. Multiple accessory dwelling units may be created within the portions of existing or proposed multifamily dwelling structures that are not used as livable space, including, but not limited to, storage

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

rooms, boiler rooms, passageways, attics, basements, or garages, if each unit complies with State building standards for dwellings.

• b. A minimum of one (1) accessory dwelling unit shall be allowed within an existing or proposed multifamily dwelling, and a maximum number of accessory dwelling units not exceeding twenty-five percent (25%) of the existing or proposed multi-family dwelling units on the parcel, if those accessory dwelling units meet all required standards of this Chapter.

2. Detached Accessory Dwelling Units on Parcels with Multi-family Structures. Up to two (2) detached accessory dwelling units shall be allowed on a parcel that has an existing or proposed multi-family dwelling, subject to the provisions of this Chapter.

1. Building permits may be issued for existing accessory dwelling units which were constructed without required permits, under the following conditions:

   • a. The accessory dwelling unit conforms to all applicable provisions of this Chapter, and all other applicable required standards for habitability.

   • b. All applicable fees for construction completed without permits have been paid.

Accessory dwelling units constructed without permits that do not meet the provisions of this Section may apply for a conditional use permit, as described in Section 8.392.110.

1. Accessory dwelling units not meeting all applicable standards of this Chapter, and which do not meet requirements of Section 8.392.120, Home Improvement Exceptions, may be conditionally permitted, subject to a conditional use permit.

2. With the exception of accessory dwelling units described in 8.392.110(3), below, the process for application for and issuance of a conditional use permit for an accessory dwelling unit shall be that set forth in Section 8.28.030 of the County Zoning Regulations, except that the granting of the permit shall be at the determination of the Zoning Hearing Officer. The determination of the Zoning Hearing Officer shall be appealable to the County Planning Commission only, subject to the procedures specified in Chapters 8.280 and Chapter 8.448 of the Zoning Regulations.

3. In the event that the creation or legalization of an accessory dwelling unit creates conflicts with standards specific to the base or overlay zoning of the parcel, or other standards for which specific exceptions are not provided in this Chapter, those conflicts must be addressed by whatever relief, if any, and through whatever procedures, are normally required by the regulations in which those standards are contained.

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

4. In the case of accessory dwelling units meeting all applicable standards of this Chapter except those related to parking requirements, a parking exception may be requested as provided in Section 8.392.05(14)(f), and a conditional use permit shall not be required.

For accessory dwelling units that may be allowed contingent on approval of a Home Improvement Exception (HIE), as described in Section 8.320.020, accessory dwelling units are exempt from the following requirements of Section 8.320.020:

1. The improvement may not result in the creation of a new story. Accessory dwelling units permitted contingent on an HIE may result in creation of a new story.

2. At least seventy-five percent (75%) of the existing exterior walls (in linear feet) will remain. Accessory dwelling units may be permitted contingent on an HIE regardless of the percent of linear feet of existing walls remaining.

3. At least fifty percent (50%) of the existing roof (in square feet) will remain. Accessory dwelling units may be permitted contingent on an HIE regardless of the percent of existing roof remaining.

4. The addition will be located at least three (3) feet from a property line. In the case of accessory dwelling units located within an existing structure, as described in subsection 8.392.050(4), accessory dwelling units may be permitted contingent on an HIE regardless of setbacks.

5. The existing structure is located in an area with an average slope of less than twenty (20) percent. Accessory dwelling units may be permitted contingent on an HIE regardless of the average slope.

The aforementioned units are also exempt from the required HIE findings of Section 8.320.060(1) and (2).

These exceptions to HIE standards are applicable only to the accessory dwelling unit, not to the primary residence or any other development on the subject parcel.

Home Improvement Exceptions may not be used to allow an accessory dwelling unit of greater floor area than that allowed by Section 8.392.050(5), or a junior accessory dwelling unit of greater floor area than allowed by Section 8.392.080(2)

San Mateo County, California, Code of Ordinances Title 8 – Zoning & Development Code

Applications for accessory dwelling units, except for those requiring a conditional use permit as specified in Section 8.392.110, shall be approved or denied ministerially, on the basis of the objective criteria included in this Chapter and other applicable regulations. Consideration of other permits associated with development of the proposed accessory dwelling unit only, that might otherwise be discretionary, including but not limited to Tree Removal, Resource Management, and Grading Permits, and Off-Street Parking Exceptions shall also be ministerial, except as provided in Section 8.392.110. No public notice or public hearing shall be required for review and approval or denial of an accessory dwelling unit, unless an applicant requests exceptions to the standards set forth in this Chapter.

Decisions to approve or deny an application for an accessory dwelling unit that meets all relevant standards set forth in this Chapter shall not be subject to appeal.

District.

These regulations shall only be applicable in areas outside San Mateo County’s Coastal Zone.

With the exception of specific standards and exemptions described in this Chapter, all accessory dwelling units must comply with all applicable provisions in the San Mateo County Ordinance Code, including the Zoning Regulations (Title 8 et seq.) and the Building Code (Title 10 et seq.).

(Chapter 22.5 - Added by Ordinance No. 2876 - January 24, 1984)

(Sections 6425 - 6434 repealed and replaced in their entirety by Ordinance No. 04768 - January 10, 2017) (Sections 6425 - 6436 repealed and replaced in their entirety by Ordinance No. 4811 - March 26, 2019) (Sections 6425 - 6437 repealed and replaced in their entirety by Ordinance No. 4834 - September 15, 2020) (Sections 6425 – 6437 repealed and replaced in their entirety by Ordinance No. 4889 – May 21, 2024)