Local zoning · San Mateo County

San Mateo County — Development Standards

Q: Do ADUs count toward density and lot coverage?

Local ADU rules in the County materials state that ADUs are exempt from maximum density limits (they do not count against district maximum density) but do count toward lot coverage ; however the County’s ADU provisions also enforce that local rules cannot be applied so as to preclude construction of at least an 800 sq ft ADU with 4 ft side/rear setbacks — see the ADU chapter (Sections 8.392.050–8.392.090) .

Development Standards under the San Mateo County local zoning and planning code, with the controlling citations.

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Last reviewed: July 3, 2026

Overview

This page explains the development-standards rules that apply in the unincorporated areas of San Mateo County under the local zoning/ordinance materials you provided. It focuses only on zoning development standards (setbacks, height, lot coverage, density/FAR and related rules as applied to districts and planned-unit regulations), not building code (Title 24), permit processing, or housing/tenant law. For related topics see the County landing pages for San Mateo County Zoning & Planning overview and specific topics like parking, design review, overlay districts, and ADUs. When the local ordinance refers to project-specific approved plans (PUDs, precise plans), those plan requirements control for that parcel (see Risks & Ambiguities).

How to read this page

Every standard below is stated for the County’s unincorporated areas.
Each firm numeric rule is cited to the local ordinance by section (for example § 8.134.120) and to the uploaded ordinance file preview. The ordinance materials provided use the County numbering that begins with “8.” (zoning/division references).
Where the uploaded materials do not contain a required item, the page says so explicitly.

District-by-district development standards

Notes: The ordinance materials provided contain multiple district chapters and project-specific PUD (Planned Unit Development) regulations. Below are the districts and chapters for which the retrieved materials include explicit dimensional standards, with plain-English interpretation and the controlling code citations.

RM (Resource Management) — general residential resource-management district

Purpose & where it applies: Resource-conservation/residential areas in unincorporated County rural and semi‑rural locations. See the RM district chapter for site-design and environmental criteria. Permitted uses are principally residential and limited supporting uses as described in the RM chapter.
Typical permitted uses: multi‑family and residential uses consistent with RM policies; project approvals tie developments to the General Plan and site-specific review. See § 8.134.040 and associated RM chapter language in the ordinance materials.
Key dimensional standards:
- Maximum height: 36 ft (3 stories) — § 8.134.120.
- Minimum front yard: 50 ft; side: 20 ft; rear: 20 ft (standard minimum yards unless reduced under specific criteria) — § 8.134.130.
- Setback reductions possible where the project preserves open space and meets the criteria in § 8.134.140 (reduction to front 20 ft, side 10 ft when criteria met).
Practical guidance: Expect Resource Management projects to require Development Review Permits and to be evaluated for open‑space tradeoffs; if you are seeking reduced setbacks, prepare the open‑space/conservation justification required by § 8.134.140.

RM-CZ (Resource Management — Coastal Zone)

Purpose & where it applies: RM rules applied to parcels inside the Coastal Zone overlay; development must meet RM standards plus coastal/Coastal Development Permit criteria. See the RM‑CZ chapter.
Typical permitted uses: similar to RM but subject to coastal-zone policy and environmental standards. All development in RM-CZ requires a Development Review Permit per the RM‑CZ chapter. § 8.294.030 and related sections establish review criteria.
Key dimensional standards (where not otherwise modified by approved plans):
- General standards follow RM but many projects are governed by the approved development plans; height and setbacks are frequently set by the approved plans or conditions of approval (see § 8.294.030 – 8.294.050).
Practical guidance: In the Coastal Zone you will usually need a Coastal Development Permit / Development Review Permit; expect site-specific plan dimensions and environmental findings to govern actual setbacks and lot coverage.

Low Density Residential designation (as used in the County’s designation tables / chapter 8.114)

Purpose & where it applies: County “Low Density Residential” designation / chapter provisions for single-family and compatible uses in unincorporated neighborhoods.
Typical permitted uses: One‑family dwellings, accessory uses consistent with residential designation; some other uses allowed by director determination.
Key dimensional standards (example items in the Low Density chapter):
- Standards and required permits are identified in 8.114.130 for Low Density Residential (see chapter text for permitted uses and any plan-specific provisions).
Practical guidance: Check § 8.114.130 for the Low Density schedule and required planning permits; many low‑density areas use standard setbacks and may allow accessory uses by Director determination.

Districts / Chapters labeled by numeric series with explicit dimensional tables (example chapter group)

The ordinance material contains multiple district chapters with explicit numeric rules in the 8.2xx and 8.13x series. Below are example provisions you will commonly encounter in the unincorporated-area regulations.

Chapter group around § 8.208 (site‑by‑site numeric standards):
- **Minimum building site width: average 50 ft — § 8.208.020.
- **Minimum lot area: 10,000 sq ft — § 8.208.030.
- **Minimum setbacks: Front 20 ft, Rear 20 ft, Side 10 ft — § 8.208.040.
- **Maximum parcel coverage: 30% — § 8.208.050.
- Floor area ratio / maximum building floor area is implemented via a table in § 8.208.060 (FAR-like floor area multipliers tied to parcel size).
Chapter group around § 8.208 / 8.208.070 also caps impervious surface area to 10% of parcel size for some residential uses (not to exceed 1,170 sq ft) — § 8.208.070.

(practical note: these numeric chapters appear when specific districts or subareas use them — verify the exact district mapping for your parcel with County zoning maps; “where it applies” detail is parcel‑specific and sometimes controlled by PUD or a precise plan).

Accessory Dwelling Units (ADUs) — special development rules that modify setbacks and density

The County’s local ADU provisions reference calculation and limits in the ADU chapter and allow ADUs to be treated differently from the standard district density limits (e.g., ADUs are exempt from district maximum density limits) — see § 8.392.XXX references in the materials.
Local ADU minimum setbacks (explicit in the uploaded materials):
- Side & rear setbacks for ADUs: 4 ft each for ADUs not entirely within existing structure; front setback may be the lesser of the district front setback or the distance of the primary residence from the front property line; stepbacks if ADU height exceeds 16 ft: side 5 ft, rear 10 ft — see § 8.392 (Sections 8.392.050–8.392.090) in the uploaded materials.
Floor area and lot coverage: the local ADU provisions require ADUs to count toward lot coverage but prohibit application of local standards in a way that precludes construction of at least an 800 sq ft ADU meeting state rules; the County references the calculation rules in Sections 8.392.050–8.392.090.
Practical guidance: ADUs have their own minimum side/rear setbacks in the County code and are explicitly treated to avoid local rules that would frustrate the State ADU law; review the local ADU chapter early and coordinate with planning staff. See statewide rules summarized in the County’s ADU references and the California ADU law portal at California ADU law.

Planned Unit Developments (PUDs) and site-specific approvals

The County has many PUD chapters (PUD‑121, PUD‑127, PUD‑126, PUD‑138, etc.). PUD provisions frequently fix height, setbacks, lot coverage, parking, and floor area to the approved plans for the parcel; in other words, for a parcel covered by a PUD the approved PUD plan/conditions often control. Examples:
- PUD‑127 (Menlo Heights Condominiums) sets maximum height 36 ft / 3 stories, and provides specific setbacks in the PUD text.
- PUD‑138 (North Fair Oaks multi‑family parcel) requires development conform to the approved plans and ties height/setback/lot coverage to those plans. See PUD excerpts.
Code citation practice: where a PUD is in effect, the PUD ordinance and its Development Plan language (the plan on file with the County and the PUD ordinance) govern; the County materials repeatedly state “development shall conform to the approved plans” in the PUD language. (See multiple PUD sections in the provided materials.)

Quick Decision‑Relevant Standards (table)

This table pulls the most commonly-used numeric standards found in the provided ordinance excerpts. Always verify the district for your parcel and whether a PUD or precise plan overrides these numbers.

District / Chapter	Max Height	Front Setback	Side Setback	Rear Setback	Lot Coverage / FAR	Code Reference
RM	36 ft (3 stories)	50 ft (std)	20 ft (std)	20 ft (std)	reductions/bonuses possible (see open space bonuses)	§ 8.134.120, § 8.134.130, § 8.134.110
RM‑CZ	Follows RM or approved plans (project plans commonly control)	Project plans or RM reductions	Project plans or RM reductions	Project plans or RM reductions	Project plans usually control	§ 8.294.030 – § 8.294.050
Chapter 8.208 (sample district chapters/tables)	Varies; building size tables apply	20 ft	10 ft	20 ft	30% parcel coverage (example) & floor area table (see table)	§ 8.208.040–§ 8.208.060
ADUs (local ADU chapter)	Attached ADU: subject to primary dwelling limit, Detached ADU: often 16–18 ft local limits (see ADU rules/state law)	Front: lesser of district or primary residence; special allowance to allow an 800 sq ft ADU	4 ft minimum (side)	4 ft minimum (rear); stepbacks if > 16 ft tall	ADUs count toward lot coverage but local rules cannot preclude an 800 sq ft ADU	§ 8.392.050–8.392.090 (local ADU chapter)

Checklist — what an applicant must satisfy (unincorporated San Mateo County)

Confirm the zoning district for the parcel and whether a PUD or precise plan applies (PUDs often control height/setbacks) — verify permit file and recorded ordinance. See the County zoning overview at San Mateo County Zoning.
Prepare site plan showing existing and proposed grades and measure building height consistent with § 8.134.120 (RM) or the controlling district §.
Show all setbacks on plan and ensure conformance with district minimums (e.g., front 20–50 ft, side 4–20 ft, rear 4–20 ft depending on district and ADU rules) — cite the district § used for each dimension.
Demonstrate compliance with lot coverage, building floor‑area table / FAR rules in the district chapter (for example § 8.208.050–8.208.060) and account for ADUs in lot coverage calculations where applicable.
Satisfy parking requirements (check the County parking chapter and references in any PUD; many PUDs reference Section 8.344.x for parking) and attach a parking plan. See San Mateo County Parking.
If in a design or overlay district, prepare materials for design review or for the applicable overlay districts; where S-17/DR combos occur, side yard reductions may require Design Review Committee approval.
If proposing an ADU, follow the local ADU chapter (setbacks 4 ft side/rear, floor area treatment, and exemptions from density limits) and consult the State ADU guidance at California ADU law.

Risks & Ambiguities

Issue	Why it matters	What to verify
PUD / approved plan overrides	Many PUDs explicitly require conformance to the approved plan and thereby supersede the generic district standards (height, setbacks, lot coverage). If a parcel is in a PUD the plan controls.	Confirm whether a PUD or precise plan ordinance applies to the parcel and read the PUD text and approved plans; PUD text repeatedly states “development shall conform to the approved plans.” (See multiple PUD sections.)
Coastal Zone / RM‑CZ special review	In the Coastal Zone RM‑CZ projects require Development Review/Coastal permits and site‑specific criteria may control setbacks/height.	Verify Coastal Development Permit / Development Review Permit requirements and the applicable RM‑CZ chapter standards § 8.294.030–8.294.050.
ADU exceptions vs. local limits	State ADU law places limits on local rules; County ADU chapter sets local setbacks but also acknowledges the state‑protected minimums (e.g., ability to build an 800 sq ft ADU).	Rely on the County ADU chapter (Sections 8.392.050–8.392.090) and confirm how lot coverage/FAR is computed for ADUs in the County file.
Which chapter applies to my parcel?	The County has multiple chapters (8.208, 8.134, 8.114, etc.) with differing numbers; applying the wrong chapter yields incorrect numeric standards.	Confirm zoning district and map, then use the specific chapter for that district (verify with County Planning). See San Mateo County Zoning.
Nonconforming parcels/structures	Existing non‑conforming situations have special rules in § 8.388 (nonconforming parcels/structures) and can affect allowable alterations.	Check § 8.388.030 and consult planning staff for whether enlargement or modification triggers a use permit.

Plain‑English summary

If your property is in an unincorporated part of San Mateo County, the County’s zoning chapters (for example the RM chapter § 8.134, chapter tables around § 8.208, and the ADU chapter § 8.392) set the default height, setback, lot‑coverage, and floor‑area rules — but many parcels are governed by site‑specific PUDs or approved plans that override the general numbers. Always check the parcel’s zoning chapter and any PUD/approved plan, and early in the project confirm parking needs and whether design review applies.

Source References

San Mateo County Zoning & Development Code excerpts (provided): § 8.134.120, § 8.134.130, § 8.134.140 (RM district height/setbacks/reduction criteria)
RM‑CZ development-review criteria and coastal standards: § 8.294.030 – § 8.294.050 (RM‑CZ District)
Sample district numeric standards, parcel coverage and floor area tables: § 8.208.020 – § 8.208.070 (building site width, area, setbacks, parcel coverage, floor area tables)
Low Density Residential designation chapter excerpts: § 8.114.130 (Low Density Residential)
ADU local provisions and setbacks / floor area treatment: Sections 8.392.050–8.392.090 (local ADU chapter excerpts in the materials)
Example PUDs and project‑specific plan language (PUD‑121, PUD‑127, PUD‑138, etc.) — PUD text routinely requires conformance to approved plans; see PUD excerpts in uploaded materials.
County parking references as used in PUDs (example cross‑references to Section 8.344.x parking standards) — see PUD excerpts that reference parking sections.

If you want, I can pull the specific ordinance text for the single parcel or district you care about (by parcel APN or address) and create a one‑page checklist tied to the controlling chapter/PUD. Verify with the County for parcel‑specific overrides.

Sources

Retrieved passages

San Mateo County Zoning Code (Section 8.114.060.) High relevance
San Mateo County Zoning Code (Title 8) High relevance
San Mateo County Zoning Code (Title 8) High relevance
San Mateo County Zoning Code (Title 8) High relevance
CBC § 66314 (§ 66314) High relevance
San Mateo County Zoning Code (Chapter 8.256) High relevance
San Mateo County Zoning Code High relevance
San Mateo County Zoning Code (SECTION A.) High relevance

Cited sections

Official San Mateo County code library

Frequently asked questions

What can I build on an RM lot in unincorporated San Mateo County?

You must follow the RM district rules in the County code: typical RM limits include a maximum building height of 36 ft / 3 stories and minimum yards front 50 ft, side 20 ft, rear 20 ft, unless a reduction is approved under the open‑space criteria; permitted uses are principally residential/low‑intensity uses — see § 8.134.120 and § 8.134.130.

What are the County’s standard setback requirements?

Setbacks depend on the district. Examples from the provided ordinance: chapter 8.208 lists front 20 ft, side 10 ft, rear 20 ft in that chapter’s application; the RM district uses front 50 ft, side 20 ft, rear 20 ft by default with reductions possible under § 8.134.140. Always verify the district and any PUD/approved plan for parcel‑specific numbers. § 8.208.040, § 8.134.130.

Are there county lot‑coverage or FAR limits?

Yes — some chapters set parcel coverage percentages (for example 30% in § 8.208.050) and other chapters implement a floor‑area table (an FAR‑style multiplier) in § 8.208.060. PUDs and approved plans may set different lot coverage or floor area rules for a parcel; where a PUD applies, the PUD text/plan controls.

Do ADUs count toward density and lot coverage?

Local ADU rules in the County materials state that ADUs are exempt from maximum density limits (they do not count against district maximum density) but do count toward lot coverage; however the County’s ADU provisions also enforce that local rules cannot be applied so as to preclude construction of at least an 800 sq ft ADU with 4 ft side/rear setbacks — see the ADU chapter (Sections 8.392.050–8.392.090).

Does the County have a single uniform height limit for all unincorporated areas?

No. Height limits are district‑specific and often project‑specific. For example, the RM district caps most structures at 36 ft / 3 stories (§ 8.134.120) while many PUDs specify a different maximum height tied to their approved plans. Always check the parcel’s zoning chapter and any PUD/approved plan.

Will my parcel’s PUD or precise plan affect setbacks and lot coverage?

Yes. The County’s PUD text repeatedly requires that development “conform to the development plans” and often sets specific heights, setbacks, lot coverage, floor area, and parking for that parcel — in those cases the PUD language and the approved plan control the standards applied to the parcel. Verify the PUD ordinance and approved plans in the County planning files.

Do I need design review for setback reductions?

If your property is within a Design Review overlay or is applying for a reduction that the code ties to design review (for example S‑17 combined with DR, or RM reductions requiring findings), you must submit to design review. The ordinance indicates side‑yard reductions in some combined S‑17/DR areas are conditioned on Design Review Committee approval (see S‑17 notes). Verify the overlay status of your parcel.

Where do I find the County’s parking rules I must follow?

Parking requirements are specified elsewhere in the zoning code and are commonly cross‑referenced in PUDs (for example, PUDs may reference Section 8.344.030 or other parking chapters). Check the County’s parking chapter and any PUD conditions; see the County parking landing page at San Mateo County Parking.

If my parcel is in the Coastal Zone, do the setbacks change?

Coastal Zone parcels are governed by RM‑CZ or Coastal Zoning chapters. The RM‑CZ chapter applies additional environmental and site‑design standards and typically requires Development Review/Coastal permits; many dimensional elements are controlled by approved plans or permit conditions rather than only the district default numbers — see § 8.294.030–8.294.050.

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