Local zoning · San Mateo County

San Mateo County — Zoning

Zoning 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 what the San Mateo County Zoning and Development Code (the County zoning ordinance that governs unincorporated areas only) says about zoning districts, maps, and the most decision‑relevant development standards. It highlights the county’s named zoning districts (for example R-E, RM, the S‑17 combining district, and site-specific PUD districts), where those rules are applied, and key rules applicants must check (setbacks, heights, lot coverage, overlay controls). For rules about parking, development standards, design review, overlay districts, ADUs, the California Building Standards Code, and historic preservation, see the linked topic pages below as you read.

All requirements listed below are for the County’s unincorporated areas and are grounded in the local ordinance text cited by section.

How the County Zoning Ordinance works (basics)

  • The ordinance establishes named zoning districts that control permitted uses, setbacks, heights, lot area and coverage, and special combining or overlay districts. The baseline rule that a site may be used only for the uses listed in its district appears in § 8.08.070.
  • Many areas also have site‑specific Planned Unit Districts (PUD‑###) or combining districts that override or add to the standard district regulations; those PUD ordinances contain the controlling, property‑specific rules (see examples PUD‑127, PUD‑138).
  • Combining and overlay districts (for example S‑11, S‑12, S‑17, and coastal or historic overlays) add standards such as minimum parcel area by slope, midcoast rules, or design review requirements — check the combining chapter applied to your parcel. See the S‑17 combining district rules at § 8.154.010.

District-by-district snapshot (where the Code gives discrete district chapters)

Below are the county districts that are well-documented in the retrieved ordinance files. Each subsection gives purpose, typical permitted uses, key dimensional standards, and where in the unincorporated county it commonly applies (when the code text says so). For any parcel-specific question, verify with the County.

R-E (Residential Estates)

  • Purpose: Large‑lot, low‑density single‑family and limited agricultural/estate uses. See § 8.12.010 for the district regulations.
  • Typical permitted uses: one‑family dwellings, public parks/playgrounds, crop/tree farming, home occupations, accessory buildings and limited guest houses (§ 8.12.020).
  • Key dimensional standards called out: minimum building site area and other development rules are set in CHAPTER 8.12 (see § 8.12.010–.020). Confirm lot‑specific minimum lot area and setbacks with County staff.

S‑17 (Combining District — Midcoast combining rules)

  • Purpose: Midcoast single‑family combining rules applied together with an R district to protect coastal/midcoast character and slope constraints. See § 8.154.010.
  • Typical permitted uses: Inherit the base single‑family district uses; S‑17 mainly modifies dimensional standards and review procedures.
  • Key dimensional rules: minimum building site width (average 50 ft), minimum building site area (5,000 sq ft), front/rear setbacks 20 ft, side setbacks 5 ft (for ≤16 ft high) or combined total 15 ft for taller structures; parcel coverage limits depend on height (50% for ≤16 ft; 35% if >16 ft) — see § 8.154.010 (3–4). The S‑17 district can be combined with a DR (Design Review) district and may allow reduced side yards subject to design review conditions.

RM (Resource Management District)

  • Purpose: Implements open‑space, conservation and resource protection policies where very low density or resource protection is essential (formerly F‑1/A‑1/A‑2 lands combined with S‑11/S‑13). See § 8.134.010–.030.
  • Typical permitted uses: Very limited residential and resource‑management uses; statute emphasizes consistency with the General Plan open‑space/conservation policies and the Local Coastal Program in the Coastal Zone.
  • Key standards and applicability: parcels 5 acres or larger generally continue as legal parcels; lots smaller than 5 acres may be legal only under narrow circumstances; maximum forms of development and resource/hazard review criteria are spelled out in § 8.134.020–.030. Verify primary resource/hazard area mapping on the County zoning map.

C‑1 / C‑1/S‑3 (Commercial — Midcoast special rules)

  • Purpose: County commercial district; when combined with midcoast S‑3, the Midcoast LCP update imposes special height and front yard options. See the Midcoast provisions in the zoning tables and the narrative around § 8.150 and midcoast notes.
  • Typical permitted uses: Commercial/retail and, where allowed, residential uses per the district text; some C‑1/S‑3 locations have specific rules for residential vs non‑residential building heights and front yard choices. See the C‑1/S‑3 notes (Midcoast) at § 8.150 and accompanying table text.

Site‑specific PUD‑### districts (planned unit districts)

  • Purpose: A PUD ordinance is a site‑specific zoning ordinance adopted for a particular property and can supersede standard district rules where it says so. Examples: PUD‑127, PUD‑138, PUD‑134, PUD‑136 (each contains a development plan, permitted uses, setbacks, heights and parking obligations). See PUD texts such as PUD‑127 (§ PUD text) and PUD‑138.
  • Typical content: exact allowed uses, number of dwelling units, maximum stories/heights (commonly 2–3 stories, 28–36 ft in examples), precise setbacks, landscaping, and concrete parking requirements (often tied to § 8.344.030 for parking rules). See the PUD examples for phrasing that PUD regulations govern where they conflict with base Part One, Division VI (Zoning).

Accessory Dwelling Units (ADUs) — county ADU rules

  • The county has a discrete ADU chapter that implements state ADU law with local rules for setbacks, lot coverage counting, height, and floor area calculations — see the ADU provisions referencing Sections 8.392.x and 8.396.x. Key local rules include minimum side/rear setbacks of 4 ft for many ADUs, maximum ADU height 26 ft, exemptions from maximum density limits, and rules that an ADU may be located within the front setback in limited circumstances to enable an 800 sq ft ADU (§ 8.392/8.396). See §§ 8.392.050–.090 and 8.396.050–.090 (ADU sections).

Quick reference table — common district standards (decision‑relevant)

District / Topic Typical permitted uses (short) Key dimensional rules (height / setbacks / lot cov.) Code reference
R‑E One‑family dwellings; parks; limited agriculture See CHAPTER 8.12 for minimum lot sizes and allowed accessory uses § 8.12.010–.020
S‑17 (Midcoast combining) Inherits base SF uses; modifies dimensions Front/Rear 20 ft; Side 5 ft (≤16 ft), combined 15 ft (>16 ft); parcel coverage 50% (≤16 ft) / 35% (>16 ft) § 8.154.010 (3–4)
RM Resource/open‑space uses, restricted residential Parcel aggregation, minimum 5‑acre rules; resource/hazard criteria § 8.134.010–.030
C‑1 / C‑1/S‑3 (Midcoast) Commercial and where allowed residential Midcoast option: special height/front yard choices (28–32 ft, variable front yard) Midcoast notes § 8.150 and table text
PUD‑### Site‑specific (multi‑family, office, mixed) PUD ordinance establishes exact height, setbacks, lot coverage and parking (overrides base rules where stated) See PUD‑127, PUD‑138 examples (PUD text)
ADUs (county chapter) Detached/attached accessory units, conversions Side/rear 4 ft minimum (stepbacks if >16 ft), max height 26 ft, floor area rules per ADU chapters; ADUs exempt from density caps Sections 8.392.x / 8.396.x

Practical guidance & interpretation tips (plain‑English implementation notes)

  • Always start at the parcel level: identify the base zoning district and any combining or overlay districts applied to your parcel on the County zoning map; base district rules are in the district chapter but combining/overlay chapters can change setbacks, coverage and review requirements (e.g., S‑17 combining rules). See the combining district rules in § 8.154.010 for S‑17.
  • If your parcel is subject to a PUD, read the PUD ordinance carefully — it frequently contains site‑specific setbacks, parking counts, and maximum height that take precedence over the general district rules (PUD‑127, PUD‑138 examples).
  • Check whether the project must meet the County’s design review or coastal review procedures — these are often triggered by combining districts or the Coastal Zone and are referenced in the district chapters (e.g., S‑17 can operate with DR reductions; see § 8.154.010 and the Development Review pointer to § 8.296). For design review background see the County’s design review page linked above.
  • Parking rules are referenced repeatedly in PUDs; off‑street parking standards are located in the County parking chapter (many PUDs refer to § 8.344.030 for off‑street parking standards). For detailed parking counts consult the County parking standards page.
  • ADUs are subject to their own chapter in the ordinance and to state ADU law; the County’s ADU sections include local setbacks, height caps (26 ft), and floor‑area calculations — review Sections 8.392.x / 8.396.x early in any accessory dwelling project.

Checklist (what an applicant must satisfy to check basic zoning conformity)

  • Identify the parcel’s base zoning district and any combining/overlay districts on the County zoning map. Verify with County staff for unincorporated parcels. (See district chapters such as § 8.12.x, § 8.154.x, § 8.134.x)
  • Confirm permitted uses in the applicable district (base + any PUD text or overlay). No use except those listed in the district is allowed per § 8.08.070.
  • Measure and confirm lot area, frontage, required setbacks, maximum height and lot coverage in the district and any combining chapter (e.g., S‑17 rules in § 8.154.010).
  • Check off‑street parking and bicycle parking standards and whether the PUD or district requires a different count; consult § 8.344.030 references in PUDs.
  • If in a combining or overlay district (S‑11/S‑12 slopes, Coastal, Historic), assemble any special studies (slope calculations per § 8.150.020, biological reports for resource areas, or historic compatibility) and note extra permit requirements (Coastal Development Permit, Historic Resource reviews).
  • For ADUs, ensure compliance with the ADU chapter rules on setbacks, height (max 26 ft), floor area calculation, and that the ADU won’t be blocked by lot coverage or density rules (ADUs are exempt from density caps) — see Sections 8.392 / 8.396.
  • Determine whether design review or Planning Commission approval is required (check the DR combining district and PUD text; see Development Review reference § 8.296).
  • For parcels with pre‑existing nonconforming uses or structures, consult the Nonconforming Uses chapter (county rules referenced to Chapter 8.428) to understand repair, enlargement or abandonment limits.

Risks & Ambiguities

Issue Why it matters What to verify
Overlapping PUD vs base district rules PUD text can supersede base district rules; relying on base rules alone may miss stricter PUD conditions Check for any PUD ordinance that applies to the parcel (search PUD‑### and the parcel APN); read the PUD text. Verify with the County.
Combining/overlay district modifications (S‑17, S‑11/S‑12, Coastal) These can change setbacks, lot areas, density and require special studies (slope, resource protection) Confirm which combining districts/overlays are mapped to the parcel and read the combining chapter (e.g., § 8.154.010, § 8.150.020).
Parcel-specific slope calculations (S‑11/S‑12) Minimum lot area and permissible dwelling units depend on slope category calculations Verify slope category and lot area calculations per § 8.150.020 and consult County GIS/mapping.
ADU applicability vs state law County ADU chapter has local limits; state ADU law may override some local rules Apply County ADU chapter Sections 8.392/8.396 and confirm interplay with state law; if ambiguous, “Verify with the jurisdiction.”
Nonconforming uses and coastal rules Nonconforming rights, and Coastal Zone restrictions, can limit alterations or require additional findings Confirm nonconforming status under Chapter 8.428 and any Coastal Development chapter 8.252 references that apply.
Parking counts and changes PUDs commonly set parking counts that differ from general tables; missing the PUD parking requirement risks non‑conformity Check PUD text and the County parking chapter (PUDs often cite § 8.344.030 for off‑street parking).

Plain‑English summary

If your property is in unincorporated San Mateo County, the County’s zoning chapters tell you exactly what uses and sizes of buildings are allowed on your lot — but you must look at the base district (for example R‑E or RM) and any combining district or site‑specific PUD that applies to your parcel because the combining chapter or PUD often changes setbacks, coverage, and parking; always confirm the parcel’s mapped designations with Planning staff before assuming a standard rule applies (see § 8.08.070).

Source References

  • Effect of establishment of districts: § 8.08.070.
  • R‑E district regulations and permitted uses: § 8.12.010–.020.
  • S‑17 Combining District (Midcoast) regulations (setbacks, coverage, floor area): § 8.154.010.
  • RM (Resource Management) district purpose and elements: § 8.134.010–.030.
  • Midcoast/C‑1 notes, slope and lot area regulations: § 8.150.020 and related Midcoast table notes.
  • Planned Unit District examples and site‑specific standards: PUD‑127, PUD‑134, PUD‑136, PUD‑138 (see individual PUD text excerpts in the ordinance file).
  • ADU local rules (setbacks, height, floor area references): Sections 8.392.x and 8.396.x (ADU chapters and notes).
  • Nonconforming uses and Historic Resource review references (Chapter 8.428, Coastal Chapter 8.252 as cited in relevant district text).
  • Parking references cited inside PUDs (off‑street parking standards often linked to § 8.344.030).

Sources

Retrieved passages

  • San Mateo County Zoning Code (Section 8.344.030) High relevance
  • San Mateo County Zoning Code (Title 8) High relevance
  • San Mateo County Zoning Code High relevance
  • San Mateo County Zoning Code (Title 8) High relevance
  • San Mateo County Zoning Code (SECTION A.) High relevance
  • San Mateo County Zoning Code (Chapter 8.428) High relevance
  • San Mateo County Zoning Code (Title 8) High relevance
  • San Mateo County Zoning Code (Title 8) High relevance

Cited sections

Frequently asked questions

What can I build on an R‑E lot in unincorporated San Mateo County?

You can build the uses listed in the R‑E chapter such as one‑family dwellings, limited agricultural uses, accessory buildings and home occupations; accessory buildings are allowed but must follow the accompanying dimensional rules in § 8.12.010–.020. Confirm minimum lot area and frontage for the specific R‑E subzone that applies to your parcel.

What are San Mateo County setback requirements for S‑17 (Midcoast) parcels?

The S‑17 combining district sets front and rear setbacks at 20 ft, side setbacks generally 5 ft for structures up to 16 ft tall, and a combined side total of 15 ft for taller buildings (with a 5 ft minimum on any one side). See § 8.154.010 (3).

Do I need design review in unincorporated San Mateo County?

Design review is required where a parcel is subject to a DR combining district, where a PUD or coastal permit requires it, or where the district chapter references Design Review procedures; the S‑17 combining language specifically contemplates DR committee approval for some side‑setback reductions. See the S‑17 rules § 8.154.010 and the Development Review pointer at § 8.296.

If my lot has steep slope, how does that affect minimum lot size and development?

Slope categories in S‑11 and S‑12 districts affect lot area requirements and permissible dwelling units; slope‑based minimums and the method to calculate average slope are in § 8.150.020 (table and formula). You must calculate slope according to the ordinance method to determine lot/parcel design standards.

Can I add an ADU to my San Mateo County unincorporated lot and what are the setback/height limits?

Yes, the County’s ADU chapter implements state ADU law with local specifications: side and rear setbacks of 4 ft for ADUs (with variations if built within existing walls), maximum ADU height 26 ft, and ADUs are exempt from density limits. See the ADU chapters (Sections 8.392.x / 8.396.x) for detailed floor‑area calculation rules and exceptions.

What happens if my property is inside a PUD?

If your parcel is in a PUD, the PUD ordinance (PUD‑###) contains the controlling development plan specifying permitted uses, parking counts, setbacks, heights and often landscaping/lighting/operations rules; where the PUD conflicts with the general zoning rules, the PUD language governs (see PUD examples PUD‑127, PUD‑138). Verify the PUD text recorded against your APN.

Are there special rules in the Coastal Zone or for historic buildings?

Yes — the County’s Coastal chapters and the historic resources chapters add additional permit and finding requirements (the County references the Coastal Development chapter 8.252 and historic Chapter 8.428 for special review and compatibility findings). Uses in historic structures may require Planning Commission findings and Historic Resources Advisory Board review.

Where do I find the parking requirements the County enforces for a project?

Parking is controlled by the County’s off‑street parking standards (many PUDs cite § 8.344.030 as the applicable parking standards). Review the parking chapter and any PUD that applies to the parcel; for high‑level guidance see the County parking topic page.

How are height limits measured in San Mateo County zoning?

The ordinance measures building height per the applicable district/plan language (PUDs commonly define height as the vertical distance from finished grade to the topmost point); example PUDs set specific height caps (for example 28–36 ft or 2–3 stories depending on the PUD). Check the relevant district or PUD language (see PUD examples and district chapters).

What if my existing use/structure doesn’t meet new zoning standards?

Nonconforming uses and structures are addressed in the ordinance; rules about maintenance, repair, and limits on enlargement are in the nonconforming uses chapter (see references to Chapter 8.428 in the ordinance). If a use lawfully existed before a new rule, nonconforming provisions may apply but there are restrictions — verify with County planning staff.

Who decides zoning interpretations and appeals in unincorporated San Mateo County?

The Zoning Administrator and Planning Commission have roles defined in the ordinance (procedural rules and hearings are described in the Development Procedures). Appeals from the Zoning Administrator go to the Planning Commission and then to the Board of Supervisors per the appeals procedures in the ordinance. See the hearing/decision language in the Development Procedures chapter. ---

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