Local zoning · San Mateo County

San Mateo County — Nonconforming Uses

Q: What is a "non-conforming use" in unincorporated San Mateo County?

A non-conforming use is any legal land use that was lawful when established but no longer fits the current zoning permit list for that district; the county defines and governs these in § 8.388.020 and regulates continuation, enlargement, abandonment and demolition in § 8.388.040 .

Q: Can I enlarge a non-conforming commercial use on my parcel?

No— non-conforming nonresidential uses may not be enlarged under the code; residential nonconforming uses are treated differently and may be enlarged only when a use permit is granted and combining-district rules are met. See § 8.388.040(5–6) .

Q: If my house (a residential nonconformity) is destroyed by fire, can I rebuild it?

If a residential non-conforming structure is destroyed by an unintended event (e.g., act of nature), the code allows rebuilding to the previous number of dwelling units and either in the same footprint or in conformance with current zoning, subject to the rules in § 8.388.050(7) . Verify site-specific limits.

Q: Do I automatically lose my non-conforming status if I stop operating my business for a while?

The code defines abandonment as voluntary cessation for 18 months . If a non-conforming use is voluntarily abandoned for that period, subsequent uses must conform; the 18-month rule and exceptions for demonstration of inability to operate are in § 8.388.020(1) and § 8.388.040(3) .

Q: What counts as “major” vs “minor” work on a non-conforming structure?

The code uses a valuation threshold: work amounting to 50% or more of the structure’s value (per current Building Valuation Data) is considered major and may trigger full conformance requirements; the definitions and the 50% rule are stated in the nonconforming chapter. See § 8.388.050(9–10) .

Q: If my lot is substandard (too small), can I still build on it?

A non-conforming parcel may remain a separate legal parcel and can sometimes be developed; whether a use permit is required depends on whether the parcel is improved/unimproved and the exact area/width thresholds in § 8.388.030 —if a use permit is required, the use-permit findings under § 8.280.030 must be made.

Q: Can the County grant exceptions to allow continuation or enlargement of a nonconformity?

Yes—the Planning Commission may grant a use permit to except provisions that restrict continuation/enlargement/reestablishment, and the Director can grant administrative exceptions in limited circumstances; however, exceptions cannot be used to exceed Midcoast maxima (floor area, height, coverage). See § 8.388.070 .

Q: Who decides if a nonconforming use should be abated for public health or neighborhood character reasons?

The Board of Supervisors , upon Planning Commission recommendation at public hearing, can require removal or conversion of a non-conforming use (except residential) when findings show detriment to public health/safety or neighborhood character; see § 8.388.040(1) . ---

Nonconforming Uses 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 summarizes how the San Mateo County Zoning & Development Code treats non-conforming parcels, uses, structures, and situations in the unincorporated areas of San Mateo County. The controlling rules are in the Zoning Nonconformities chapter (Chapter 8.388) and related provisions (for example § 8.300.080) and distinguish residential nonconformities (generally preserved) from nonresidential nonconformities (phased out or limited). See the county’s development standards for project-level dimensional rules and verify parcel-specific rules with the County.

Key rules (plain-English synthesis, code grounding)

Continuation: A legal non-conforming use, structure, parcel, or situation may generally continue provided it meets the Chapter 8.388 rules and does not create a public nuisance; confined-animal uses that degrade habitat or water quality can be abated. See § 8.388.040 (uses), § 8.388.050 (structures), § 8.388.030 (parcels), and § 8.388.060 (situations).
Repair / Remodel: The code separates minor vs major work. Minor work (≤50% of structure value) can generally be done without forcing full zoning conformance; major work (≥50% of structure’s value) may trigger a requirement that the entire structure conform if any nonconformity exceeds 50% of the required standard. See § 8.388.050(5) and definitions of major/minor in the chapter.
Enlargement: Non-conforming nonresidential uses may not be enlarged; residential nonconforming uses (for example, a residence in an industrial zone) can sometimes be enlarged but typically require a use permit and must follow specified combining district rules (S-3, S-5, S-7, S-17). See § 8.388.040(5–6) and the combining-district table.
Abandonment / Reestablishment: “Abandoned” is defined as voluntary cessation for at least 18 months. If a non-conforming (nonresidential) use is abandoned, subsequent uses must conform; residential nonconforming uses have more flexibility to be reestablished. See § 8.388.020(1) and § 8.388.040(3–4).
Destruction / Rebuilding: If a non-conforming nonresidential structure/use is destroyed, replacement must conform to current zoning. Residential nonconforming structures destroyed by acts of nature may be rebuilt to the previous number of units and either in the prior footprint or in conformance with the code; other demolitions/replacements must conform. See § 8.388.050(6–7) and § 8.388.040(9–10).
Non-conforming parcels: A legal parcel that is substandard (area, width or frontage) may remain separate but development on an unimproved or improved non-conforming parcel can require a use permit depending on size/width thresholds and other conditions. Use-permit findings are required under § 8.280.030. See § 8.388.030.
Exceptions & Variances: The Planning Commission can grant a use permit to except the Chapter’s restrictions; the Director can grant administrative exceptions when required to comply with another government mandate. Note a special constraint: no use permit may be used to exceed maximum floor area, height, or parcel coverage in the Midcoast area. See § 8.388.070. Link to county variances and exceptions.
Situations (parking, signs, landscaping): Non-conforming situations (e.g., parking that does not meet current standards) can continue, be repaired, or be enlarged only as long as the enlargement meets current standards (for example, parking and sign regulations). See § 8.388.060 and consult county parking rules for detail.
Relation to building code: Nonconforming zoning relief does not override the California Building Standards Code — structural safety/ flood-standards requirements must still be met when repairing, rebuilding, or doing major work (references in § 8.300.080).

District-by-district guidance (unincorporated San Mateo County)

Note: This section summarizes the nonconforming treatment and the specific district labels that the nonconforming chapter itself references. For parcel-specific dimensional numbers (setbacks, lot coverage, heights) consult the county development standards and the zoning map (Verify with the jurisdiction).

Residential (R) districts

Purpose and scope: Residential (R) districts are where residential uses (one-family, two-family, multiple-family, ADUs) are the primary permitted use; residential uses are defined in § 8.388.020(16). Residential nonconformities are explicitly favored for continuation to preserve housing. See § 8.388.010 and the residential definitions.
Typical permitted uses: One-family and multi-family dwellings, accessory uses (including ADUs — see state rules and the county’s ADU chapter). Link to California ADU law: California ADU law.
Nonconforming treatment: Residential non-conforming uses may be repaired, substantially upgraded, reestablished after abandonment in many cases, and (in some cases) rebuilt after destruction subject to limits on unit count and location. See § 8.388.040(4,8,10) and § 8.388.050(7).
Dimensional standards: Specific setbacks/heights/coverage for each R district are not listed in the nonconformities chapter; consult Development Standards. Not found in retrieved materials.

Combining zoning districts: S-3, S-5, S-7, S-17

Purpose: These combining-district designations are referenced when the code regulates how residential nonconformities in nonresidential zones may be enlarged or rebuilt; the code ties One Family, Two Family, and Multiple Family use allowances to these combining district regulations. See § 8.388.040(6) and § 8.388.050(10–11).
Typical effect: If a residence sits in a non-residential zone, enlargement or replacement can be conditioned to conform to the combining-district regulations (for example, S-17 inside coastal zone for one-family, S-7/S-5/S-3 outside certain zones). See the combining-table in § 8.388.040/050.
Dimensional standards & applicability: The chapter references the combining districts but does not reproduce full numeric standards for each S-district in the nonconforming chapter; consult the base zoning district text and development standards. Not found in retrieved materials.

Midcoast (overlay area)

Purpose and limits: The Midcoast overlay receives special protection. The code explicitly prohibits granting use permits that would allow exceedance of maximum floor area, height, and parcel coverage for Midcoast parcels when considering exceptions to the nonconforming chapter. See § 8.388.070(1). Link: Overlay Districts.
Practical effect: Even when the Planning Commission can grant exceptions for nonconformities, Midcoast maxima remain binding.

Planned Unit Development (PUD) and specific PUD ordinances

Purpose and effect: PUD ordinances (e.g., PUD‑124, PUD‑125 in the code examples) set site‑specific permitted uses and dimensional rules that can supersede general code provisions for that parcel. If your lot is within a PUD, consult the text of that PUD ordinance in the Code. Examples in the zoning code show permitted uses, density and height caps specific to those PUDs. See example PUD ordinance excerpts.
Nonconforming effect: A PUD’s specific provisions control development when they conflict with general rules — verify with the County.

Quick decision table (most decision-relevant standards)

Issue	What the code allows/limits (plain-English)	Primary Code Reference
Continue an existing non-conforming use	Generally allowed to continue; confined-animal uses that degrade habitat/water may be abated.	§ 8.388.040
Repair / Remodel (structure)	Minor (≤50%) allowed; major (≥50%) may require full conformance if any nonconformity is ≥50% of standard.	§ 8.388.050(5) and definitions (major/minor)
Enlarging a non-conforming nonresidential use	Not allowed.	§ 8.388.040(5)
Enlarging a non-conforming residential use in non-residential zone	May be allowed with a use permit and must meet combining-district rules (S‑3/S‑5/S‑7/S‑17).	§ 8.388.040(6)
Abandonment (reestablish)	Defined as voluntary cessation ≥18 months; nonresidential abandoned uses must conform; residential may be reestablished.	§ 8.388.020(1) and § 8.388.040(3–4)
Destruction / replacement	Nonresidential: must conform if destroyed; Residential: may be rebuilt subject to limits and use-permit requirements for residences in nonresidential zones.	§ 8.388.050(6–7) and § 8.388.040(9–10)
Exceptions / Use permits	Planning Commission may grant use-permit exceptions; Director may grant administrative exceptions in limited cases; no exceptions to exceed Midcoast maxima.	§ 8.388.070

Checklist (what an applicant must do / verify)

Confirm the property is in unincorporated San Mateo County and identify its exact base district and any overlays (Midcoast, PUD, etc.). Verify with County maps.
Determine whether the condition is a non-conforming parcel, use, structure, or situation using § 8.388.020 definitions.
If proposing development on a non-conforming parcel, confirm whether a use permit is required per § 8.388.030 and prepare the required use‑permit findings (see § 8.280.030).
For repairs/remodels, calculate the valuation percentage (major vs minor) per the chapter’s valuation rule; if ≥50%, plan for full conformance or justify otherwise.
If the project involves parking, signs, landscaping, or other site items, consult the county parking and landscaping and screening standards and confirm nonconforming-situation rules.
If the site is in the Midcoast, do not rely on a use permit to exceed floor area, height, or parcel coverage maxima. Verify.
If historic or floodplain issues apply, prepare technical documentation and evaluate exception procedures (see § 8.300.090).
Coordinate plan compliance with the California Building Standards Code for structural or flood-safety upgrades.

Risks & Ambiguities

Issue	Why it matters	What to verify
Is the use “abandoned”?	Abandonment (≥18 months) converts status—nonresidential abandonment forces conformance.	Confirm continuous use history, leases, permits, and operation records; see § 8.388.020(1).
Major vs Minor repair valuation	Crossing 50% valuation can force full conformance (costly).	Have Building Division/estimator calculate work as defined in the chapter and reference Building Valuation Data method. See § 8.388.050(9–10).
Which combining S‑district applies?	Determines whether a residential nonconformity in a nonresidential zone may be enlarged or rebuilt.	Verify combining district on parcel (S‑3/S‑5/S‑7/S‑17) and apply the table in § 8.388.040/050.
Midcoast overlay caps	Even if exception otherwise allowed, Midcoast maxima cannot be exceeded by permit.	Confirm overlay boundaries and Midcoast maxima; see § 8.388.070.
Parcel-specific numeric standards (setbacks, coverage)	Nonconformity outcomes often hinge on exact numeric offsets.	Not found in the nonconformities chapter—pull the base district numeric standards in the Development Standards and zoning district text. Verify with the County. Not found in retrieved materials.
Conflicts with other laws (ADU/state law)	State ADU law limits local ability to deny ADUs because of nonconforming zoning conditions.	Cross-check state ADU rules and county ADU chapter; see § 8.392 and state ADU law (link to California ADU law).

Plain-English Summary

If your property in unincorporated San Mateo County doesn't meet current zoning, you usually can keep and repair what already exists—especially for homes—but enlarging or replacing nonresidential uses is tightly limited and may require a use permit; destroyed nonresidential buildings generally must be rebuilt to current rules while residential rebuilding has more flexibility. Verify overlay rules (Midcoast), combining-district rules (S-3/S-5/S-7/S-17), and whether your proposed work counts as a “major” rebuild under the code.

Source References

San Mateo County Zoning & Development Code — Chapter 8.388 (Zoning Nonconformities), §§ 8.388.010–8.388.070 (purpose, definitions, parcels, uses, structures, situations, exceptions).
San Mateo County Zoning & Development Code — § 8.388.030 (Non-Conforming Parcels) (development and use-permit triggers).
San Mateo County Zoning & Development Code — § 8.388.040 (Non-Conforming Uses) (continuation, enlargement, abandonment, destruction).
San Mateo County Zoning & Development Code — § 8.388.050 (Non-Conforming Structures) (repairs, enlargement, destruction/rebuilding rules, 50% valuation test).
San Mateo County Zoning & Development Code — § 8.388.060 (Non-Conforming Situations) (parking/sign/landscape nonconformities).
San Mateo County Zoning & Development Code — § 8.388.070 (Exceptions; Planning Commission/Director authority; Midcoast limits).
Related: § 8.300.080 (non-conforming uses within another Chapter; flood/other exceptions) and § 8.300.090 (exceptions for historic/flood areas).
Examples of site-specific PUD ordinances in the code (PUD‑124, PUD‑125) that illustrate PUD-level permitted uses and numeric caps; see the PUD excerpts in the Code.

Sources

Retrieved passages

CBC § 8.280.030 (Section 8.280.030) High relevance
CBC § 388.040 (Title 8) High relevance
CBC § 388.060 (Chapter are) High relevance
CBC § 388.030 (Title 8) High relevance
CBC § 388.050 (Title 8) High relevance
CBC § 8.280.030 (Chapter are) High relevance
CBC § 388.050 (Chapter are) High relevance
San Mateo County Zoning Code (Chapter but) High relevance

Cited sections

Official San Mateo County code library

Frequently asked questions

What is a "non-conforming use" in unincorporated San Mateo County?

A non-conforming use is any legal land use that was lawful when established but no longer fits the current zoning permit list for that district; the county defines and governs these in § 8.388.020 and regulates continuation, enlargement, abandonment and demolition in § 8.388.040.

Can I enlarge a non-conforming commercial use on my parcel?

No—non-conforming nonresidential uses may not be enlarged under the code; residential nonconforming uses are treated differently and may be enlarged only when a use permit is granted and combining-district rules are met. See § 8.388.040(5–6).

If my house (a residential nonconformity) is destroyed by fire, can I rebuild it?

If a residential non-conforming structure is destroyed by an unintended event (e.g., act of nature), the code allows rebuilding to the previous number of dwelling units and either in the same footprint or in conformance with current zoning, subject to the rules in § 8.388.050(7). Verify site-specific limits.

Do I automatically lose my non-conforming status if I stop operating my business for a while?

The code defines abandonment as voluntary cessation for 18 months. If a non-conforming use is voluntarily abandoned for that period, subsequent uses must conform; the 18-month rule and exceptions for demonstration of inability to operate are in § 8.388.020(1) and § 8.388.040(3).

I want to convert an old, nonconforming garage into an ADU — can the County deny it because the property is nonconforming?

State ADU law limits the County’s ability to deny ADU permits because of nonconforming zoning conditions; the County’s ADU chapter and state rules apply. Consult the County ADU chapter and state ADU law; see § 8.392 and the state ADU references. Link: California ADU law.

What counts as “major” vs “minor” work on a non-conforming structure?

The code uses a valuation threshold: work amounting to 50% or more of the structure’s value (per current Building Valuation Data) is considered major and may trigger full conformance requirements; the definitions and the 50% rule are stated in the nonconforming chapter. See § 8.388.050(9–10).

If my lot is substandard (too small), can I still build on it?

A non-conforming parcel may remain a separate legal parcel and can sometimes be developed; whether a use permit is required depends on whether the parcel is improved/unimproved and the exact area/width thresholds in § 8.388.030—if a use permit is required, the use-permit findings under § 8.280.030 must be made.

Can the County grant exceptions to allow continuation or enlargement of a nonconformity?

Yes—the Planning Commission may grant a use permit to except provisions that restrict continuation/enlargement/reestablishment, and the Director can grant administrative exceptions in limited circumstances; however, exceptions cannot be used to exceed Midcoast maxima (floor area, height, coverage). See § 8.388.070.

Where do I find the numeric setback, height, and coverage standards that my nonconforming project must meet?

Those numeric development standards are in the base zoning district rules and the county development standards. The nonconforming chapter defers to those standards and references them when requiring that replaced or enlarged portions conform. Numeric specifics are not reproduced in Chapter 8.388. Not found in retrieved materials; verify with the County.

Who decides if a nonconforming use should be abated for public health or neighborhood character reasons?

The Board of Supervisors, upon Planning Commission recommendation at public hearing, can require removal or conversion of a non-conforming use (except residential) when findings show detriment to public health/safety or neighborhood character; see § 8.388.040(1). ---

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