San Mateo — Nonconforming Uses

Last reviewed: July 2, 2026

Overview

This page explains how the City of San Mateo treats nonconforming uses, nonconforming structures, and nonconforming lots under the San Mateo Zoning Code (Title 27). The controlling rules are in Chapter 27.72 (Nonconforming Buildings and Uses) and apply citywide, with a few district-specific exceptions and cross-references in the residence, commercial, transit, and special-district chapters. Key practical limits are discontinuance/abandonment, alteration/destruction thresholds, enlargement rules, and the option to convert a nonconforming use to a special use permit. See the underlying code in § 27.72.010 – § 27.72.100 for the full legal text.

(For zoning district maps and permitted uses consult the city's San Mateo Zoning pages; for dimensional rules consult San Mateo Development Standards.)

How San Mateo’s nonconforming rules work — essentials by code

• Legal nonconforming uses and structures that lawfully existed when Title 27 took effect may continue, but their continuation, repair, enlargement, rebuilding after damage, and reestablishment after discontinuance are strictly limited by Chapter 27.72. § 27.72.010 (Continuance) is the starting point.

• Discontinuance/abandonment: a nonconforming use that is discontinued for six consecutive months may not be reestablished; the property must thereafter conform to the current district regulations. § 27.72.020(b–c).

• Damage/destruction threshold: if a nonconforming building is damaged or destroyed to 50% or more of its replacement value, it may not be rebuilt or used again for the nonconforming use — except limited statutory exceptions for some residential, commercial, and TOD structures. § 27.72.070(a–b).

• Repairs/alterations: ordinary maintenance is allowed; structural alterations that would extend or intensify the nonconforming use are generally prohibited unless required by law or eliminate the nonconforming use. § 27.72.060(a–b).

• Enlargement/additions: a nonconforming building may be enlarged only if the entire building becomes conforming and then meets all district regulations; you cannot expand the nonconforming portion. § 27.72.080(a–d).

• Exemptions: limited exemptions exist (for example, where the only nonconformity is number of dwelling units in an R district, or certain C- or M-district proximity issues). See § 27.72.090.

• Conversion to a special use: a property owner may seek to regularize a nonconforming use by applying for a special use permit under Chapter 27.74; Chapter 27.74 explicitly contemplates nonconforming uses as candidate special uses. § 27.72.100; Chapter 27.74.

Below is a decision-focused table of the most-used standards to check immediately.

District-by-district breakdown (how nonconforming rules interact with specific San Mateo districts)

The nonconforming rules in Chapter 27.72 apply across the city, but the underlying district chapters set the uses and bulk rules that determine what is "nonconforming" in the first place. Below are the key districts where city code language interacts with nonconformity in ways applicants commonly encounter.

R districts (Residence: e.g., R-1-A, R-1-B, R-C)

Purpose and where it applies

• The R districts regulate single- and multi‑family residential development across much of the city; see Chapter 27.16 for cross references and Chapter 27.18 for bulk standards. § 27.16.010 cross-references Chapter 27.72.

Typical permitted uses

• Single-family and multi-family dwellings and permitted accessory uses per residence chapters (see Title 27 residential articles). Verify the exact permitted list for each R subtype in the Code. Not all R subzones are listed verbatim in the retrieved excerpts. Verify with the San Mateo Zoning page.

Key dimensional standards (examples)

• Rear yard minimum usually 15 ft; new construction over one story often 25 ft for rear yard. § 27.18.090.
• Yards and permitted projections are explained in § 27.18.100.

How nonconforming rules play out here

• Structural-damage exceptions: residential structures in residential districts are explicitly exempted from the 50%-destruction rebuild prohibition — meaning you may restore residential buildings following partial or total damage under the circumstances described in § 27.72.070(b). Verify whether your residence satisfies the exemption conditions.
• R-district specific exemption: buildings nonconforming only by number of dwelling units are specially treated (no increase allowed). § 27.72.090(1).

(If an ADU is involved, state ADU law may also limit what the City can condition — see San Mateo ADUs and state guidance.)

C districts (Commercial: e.g., C-1, other C categories)

Purpose and where it applies

• The City’s commercial districts house neighborhood, community, and regional retail/office uses (see the zoning chapters for the specific C-district hierarchy). Nonconforming commercial uses are governed by Chapter 27.72.

Typical permitted uses

• Retail, office, personal services; specifics vary by C subtype. (Consult San Mateo Zoning.)

How nonconforming rules play out here

• Damage/destruction: the 50% threshold restriction does not apply to certain commercial buildings (see the exceptions list in § 27.72.070(b)). Confirm whether your building is listed as exempt.
• Proximity nonconformities (e.g., a commercial use closer to an R district than allowed) may be recognized as exempt in limited cases per § 27.72.090(3).

M districts (Manufacturing / Industrial)

Purpose and where it applies

• Industrial/manufacturing uses and related services; see the M-district articles in Title 27.

How nonconforming rules play out here

• Proximity and bulk nonconformities (parking, loading, yards, FAR, lot area) are specifically listed in § 27.72.090(4) as categories where a building may be nonconforming and yet be exempt from some repair limits — but this is a narrow administrative list.

TC — Transportation Corridor District (the TC district)

Purpose and where it applies

• The TC district is intended to protect highway and rail corridors and restrict encroaching development (see § 27.73.010).

Typical permitted uses

• Railroad and freeway rights-of-way, public utilities, transit stations, accessory parking and limited retail for passengers. § 27.73.020 lists permitted uses.

How nonconforming rules play out here

• Nonconforming uses that interfere with corridor function may be subject to termination or limitations under Chapter 27.72 in the public-interest balancing called for in the TC district intent. See § 27.73.040 for conditions.

S — Shoreline / Water-Edge District

Purpose and where it applies

• The S district governs shoreline uses and is subject to special performance standards and buffers. § 27.59.040–.070 set conditions, setbacks, and parking expectations.

How nonconforming rules play out here

• Nonconforming shoreline uses may have additional public‑access, buffer, or other site-improvement conditions that affect whether a nonconforming use can be continued, enlarged, or rebuilt under Chapter 27.72. See the S-district conditions for details.

TOD — Transit-Oriented Development Districts and Rail Corridor

Purpose and where it applies

• TOD districts implement the Rail Corridor Plan; development standards are set by the Rail Corridor Plan but remain subject to Title 27 where not in conflict. § 27.90.050–.070.

How nonconforming rules play out here

• Note that § 27.72.070(b) lists TOD district buildings among those exempted from the 50% damage rebuild prohibition in certain circumstances — confirm applicability if your structure is within a TOD area.

Overlay and Qualified Districts (examples: Two‑Unit Development Overlay, Q6/Q7/Q8)

Purpose and where it applies

• Overlay districts add special rules on top of underlying zones: the Two‑Unit Development Overlay (SB 9 implementation) is in Chapter 27.21; qualified overlays Q6, Q7, Q8 appear in other special‑district chapters. These overlays can change allowable uses, heights, FAR, and approvals. See Chapter 27.21 and overlay descriptions.

How nonconforming rules play out here

• Overlays change what is conforming. A use that became nonconforming when the overlay was adopted remains governed by Chapter 27.72 unless the overlay or a specific map amendment changed the underlying entitlement; owners may pursue reclassification or a special use permit if needed. For Two‑Unit Overlay / SB 9 projects, objective design and lot-split rules can interact with existing nonconforming conditions — verify with Chapter 27.21.

Checklist — what an applicant must satisfy / confirm before acting on a nonconforming property

• Establish that the use or structure was lawfully existing at the effective date of Title 27 (legal nonconforming). § 27.72.010.
• Confirm the downtime / discontinuance history (no continuous 6 months of abandonment). § 27.72.020(b–c).
• If damage occurred: obtain a valuation showing whether destruction ≥ 50% of replacement value; if ≥50%, rebuilding for the nonconforming use is generally barred (exceptions exist for certain residential/commercial/TOD buildings). § 27.72.070(a–b).
• For proposed repairs or alterations, document that changes are nonstructural or otherwise permitted (alternatively, show the alteration eliminates the nonconformity). § 27.72.060.
• For additions/enlargements: demonstrate the entire building will be made conforming and meet all district regulations (or redesign to remove the nonconforming portion). § 27.72.080.
• Check for narrow exemptions that may apply (e.g., unit-count in R districts; proximity in C/M districts). § 27.72.090.
• If regularization is desired, prepare a special use permit application under Chapter 27.74 (public hearing required). § 27.72.100 and Chapter 27.74.
• Coordinate with design and review procedures where additions or SPAR are required (San Mateo Design Review). § 27.08.030 cross-references.
• Confirm off-street parking, loading, and other development standards (consult San Mateo Parking and Chapter 27.64). § 27.72.090(4) discusses parking as a nonconforming category.

Risks & Ambiguities

Plain-English Summary

If your building or business in San Mateo was legal under the old rules but no longer meets today's zoning, you usually can keep using it—but you cannot abandon it for six months, cannot make it bigger in the nonconforming area, and if it's more than half destroyed you may lose the right to use it as before. Some narrow exceptions exist for certain residential, commercial, and transit-area buildings, and you can pursue a special use permit to legalize the use. The controlling rules are in Chapter 27.72 of Title 27.

Source References

• San Mateo Municipal Code — Chapter 27.72: Nonconforming Buildings and Uses (see § 27.72.010 – § 27.72.100).
• San Mateo Municipal Code — § 27.72.060 (Repairs/Alterations) and § 27.72.070 (Damage/Destruction).
• San Mateo Municipal Code — § 27.72.090 (Exemptions) and § 27.72.100 (Conversion to Special Use).
• Residence district cross references: § 27.16.010 and residential bulk rules § 27.18.090–100.
• Transportation Corridor district: § 27.73.010–020.
• Shoreline/S district conditions: § 27.59.040–.070.
• TOD / Rail Corridor plan cross references: § 27.90.050–.070.
• Two‑Unit Development Overlay (SB 9) / Chapter 27.21 (overview and interaction notes).
• San Mateo special overlay examples Q6/Q7/Q8 (qualifications and limits).
• State ADU guidance (context where ADUs interact with nonconforming zoning): uploaded California ADU handbook (useful interpretation).

Retrieved passages

• San Mateo Zoning Code (title becomes) High relevance show source text
• San Mateo Zoning Code (title may) High relevance show source text
• San Mateo Zoning Code (Section 27.08.090) High relevance show source text
• San Mateo Zoning Code (title may) High relevance show source text
• San Mateo Zoning Code (Chapter 27.74.) High relevance show source text
• San Mateo Zoning Code (Chapter 13.40.) High relevance show source text
• CBC § 66321 (§ 66321) Medium relevance show source text
• CFC § 65589.5 (Section 65589.5) Medium relevance show source text

Cited sections

• San Mateo Municipal Code — Chapter **27.72**: Nonconforming Buildings and Uses (see **§ 27.72.010 – § 27.72.100**). (§ 27.72.010)
• San Mateo Municipal Code — **§ 27.72.060** (Repairs/Alterations) and **§ 27.72.070** (Damage/Destruction). (§ 27.72.060)
• San Mateo Municipal Code — **§ 27.72.090** (Exemptions) and **§ 27.72.100** (Conversion to Special Use). (§ 27.72.090)
• Residence district cross references: **§ 27.16.010** and residential bulk rules **§ 27.18.090–100**. (§ 27.16.010)
• Transportation Corridor district: **§ 27.73.010–020**. (§ 27.73.010)
• Shoreline/S district conditions: **§ 27.59.040–.070**. (§ 27.59.040)
• TOD / Rail Corridor plan cross references: **§ 27.90.050–.070**. (§ 27.90.050)
• Two‑Unit Development Overlay (SB 9) / Chapter **27.21** (overview and interaction notes).
• San Mateo special overlay examples **Q6/Q7/Q8** (qualifications and limits).
• State ADU guidance (context where ADUs interact with nonconforming zoning): uploaded California ADU handbook (useful interpretation).
• SanMateo_ZoningCode.md
• 2025 California ADU handbook.md

• Official San Mateo code library