Mill Valley — Nonconforming Uses

Last reviewed: July 2, 2026

Overview

This page summarizes what the City of Mill Valley's Zoning Ordinance (Title 20) says about nonconforming uses, nonconforming buildings or structures, and how those rules interact with district development standards, demolition/rebuild, and re‑establishment after abandonment. It is a code-focused synthesis (plain language) — linkouts point you to the city's planning/zoning topics such as parking, design review, ADUs, development standards, overlay districts and the California Building Standards Code where the ordinance refers to building/occupancy classification. Definitions and base rules live in the ordinance definitions chapter and Title 20 chapters cited below. See "Information Gaps" if you need the exact local section number for the ordinance text that defines the core nonconforming rules.

What the Mill Valley ordinance says (core rules)

• Definitions: Title 20 defines a nonconforming building or structure and nonconforming use in the ordinance definitions chapter (Chapter 20.08). The code treats a nonconforming building as either (A) a building designed for a nonconforming use, or (B) a building that does not comply with Title 20 development standards (height, yards/setbacks, adjusted floor area, lot coverage). See Chapter 20.08 (Definitions) for the definitions and related text.

• Continued lawful operation: A lawful nonconforming use existing when the ordinance or an amendment took effect may continue but may not be enlarged, increased, or extended to occupy a larger area than it originally occupied; it also must not be operated in a manner that constitutes a public nuisance. (Text in Title 20 describing continuation of longtime lawful uses.)

• Abandonment / discontinuation: If a nonconforming use or occupancy is abandoned or discontinued for a continuous period of one year, any later use must conform to the current zoning for the district. Shorter cessations (less than one year) are not automatically abandonment, and a property owner who, within one year after cessation, applies for and diligently pursues permits to re-establish the use can avoid abandonment. (Ordinance text on abandonment / re-establishment.) Not all subsections are labeled with a local § number in the retrieved materials.

• Parking when re‑establishing: Parking requirements for re‑established nonconforming uses are the same as for newly established uses; the code cross-references the off‑street parking rules at § 20.60.090.

• Replacement after demolition (voluntary vs. involuntary): The ordinance includes clear thresholds for reconstruction after demolition of a nonconforming building: a voluntary demolition that destroys more than 50% of exterior roof or walls (or combined area of roof + walls) within any 24‑month period requires reconstruction in compliance with current Zoning Ordinance standards; an involuntary demolition threshold is higher at 75%, but involuntary demolition may allow rebuilding to the same square footage for buildings that exceed FAR limits while requiring conformance with all other standards. The demolition/rebuild language and thresholds appear in the ordinance text retrieved (see citations), but an exact local section number for this nonconforming-demolition text was not clearly labeled in the retrieved excerpt.

• Minor remodeling and exceptions: Minor maintenance and repairs are allowed on nonconforming buildings (painting, window/roof repair, fixture replacement) provided they comply with applicable provisions. The code expressly allows certain exceptions for safety/code upgrades (seismic, Title 24 / Building Code upgrades), limited floodplain modifications, and voluntary code upgrades limited to bringing a building closer into compliance — these are allowed without triggering full conformance in some cases.

• Occupancy classification: A nonconforming structure or area of a structure must remain within the same occupancy classification as defined by the most recent version of the California Building Standards Code (i.e., changes in occupancy class generally require full compliance).

• Conversion or similar use in residential zones: If a nonconforming commercial building sits in a residential district but still has economic life as commercial, the Planning Commission may allow other similar commercial uses via a conditional use permit limited to the building’s useful economic life (must find no detriment to the residential area).

Important code cross-references cited by the ordinance text include § 20.60.090 (off‑street parking), Chapter 20.90 (accessory dwelling / second units), and Section 20.66.045 / 20.66.030 (design review exemptions and limits) — those cross‑references are used by the nonconforming provisions to allocate related procedural and technical requirements.

District-by-district breakdown (how nonconforming rules interact with specific Mill Valley zoning districts)

The ordinance organizes permitted uses and development standards by districts. Below are district‑specific notes focused on how the nonconforming rules in Title 20 interact with each district’s standards (purpose, typical permitted uses, key dimensional numbers, and where the district applies). Each bullet cites the ordinance material retrieved.

Note: For general district permitted uses and standards the ordinance uses several chapters and "Use Tables" (for example, see the Use Table in § 20.24.030). Verify parcel‑level rules with the City.

RM‑3.5 (Multi‑Family Residential — 3.5)

• Purpose & where it applies: Intended for multifamily housing at prescribed densities including special Miller Ave and Bayfront subareas; Bayfront includes named developments (Bayfront, Enchanted Knolls, Shelter Bay, Shelter Hill, Plymouth Square).
• Typical permitted uses: single‑family dwelling, multi‑family dwellings, accessory structures, small residential facilities; some nonresidential uses are allowed only with CUP.
• Key dimensional standards (decision‑relevant): Min land area per unit = 3,500 s.f., Max lot coverage = 50%, Max building height = 35 ft (transitional limits near single‑family lots reduce to 25 ft), Exterior/front setback for habitable structures = 15 ft (see multi‑family tables).
• How nonconforming rules apply: Existing single‑family dwellings in RM zones are explicitly not considered "nonconforming" if existing at adoption and may be altered/enlarged if additions meet setback/height/other standards (so single-family homes in those MF zones retain more flexibility than commercial nonconforming uses).

DR (Downtown Residential)

• Purpose & where it applies: Downtown residential character area; used to manage residential forms in the city center.
• Typical permitted uses: single‑family dwellings, second units / accessory units, multi‑family (with standards), home occupations; nonresidential uses are often conditional (CUP).
• Key dimensional standards: Max lot coverage = 50%, FAR (multi) = 0.60 (varies by subrules), height = 25 ft at initial setback, may increase to 35 ft; front setback varies by slope tables used citywide.
• How nonconforming rules apply: The code states existing single‑family residences at adoption are treated as conditionally permitted and not "nonconforming" subject to RS‑6 standards when altered; other nonconforming commercial/residential uses must follow the general nonconforming rules (abandonment, demolition thresholds, rebuild rules).

RS‑6 (Single‑Family Residential zones; e.g., RS, RSP, RMP variants)

• Purpose: Traditional single‑family neighborhoods; development standards are in Chapter 20.16 (see § 20.16.040 for minimum setbacks / adjusted floor area rules).
• Typical permitted uses: single‑family dwellings, accessory structures, home occupations, ADUs (Chapter 20.90).
• How nonconforming rules apply: Single‑family homes existing at ordinance adoption are often protected from being labeled nonconforming in some mixed/multi zones (see RM notes). Any change that intensifies occupancy or changes occupancy classification must comply with building/occupancy rules; accessory structures/conversions must meet accessory rules (setbacks, height, separation). See definitions and accessory structure standards in Chapter 20.08 and accessory sections.

C‑G / C‑D / C‑N (Commercial districts — General, Downtown, Neighborhood)

• Purpose & where they apply: Commercial uses across the city (downtown, corridors, neighborhood centers), regulated by Chapter 20.40 and subsequent district chapters.
• Typical permitted uses: Retail, restaurants, offices (many subject to CUP), mixed‑use with residential above, parking requirements cross‑referenced to § 20.60.090.
• How nonconforming rules apply: Commercial uses that become nonconforming (e.g., retail that doesn't meet new district rules) may continue but may not expand beyond their pre‑existing footprint; if abandoned for one year it loses nonconforming status; special provisions allow the Planning Commission to consider CUPs for similar commercial uses when a commercial building sits in a residential district but still has economic life. Parking re‑establishment requirements are tied to § 20.60.090.

R‑P (Resource/Planned)

• Purpose & where applies: Larger parcels requiring master planning; development regulated through PUD/master plan procedures (Ch. 20.18). Uses are limited until plan approval. Nonconforming use rules still apply to any lawful uses pre‑dating regulations; planned development approvals can allow deviations.

(For full lists of permitted/conditional uses and detailed dimensional tables see the Use Table and district chapters; the code cross‑references sections such as § 20.24.030 for use tables and Chapter 20.16 for RS standards.)

Decision‑relevant table: Selected nonconforming rules and code cross‑references

Checklist — What an applicant must satisfy before pursuing a nonconforming re‑establishment or rebuild in Mill Valley

• Confirm whether the building/use is listed as a lawful nonconforming use in Title 20 (see definitions in Chapter 20.08).
• If use was discontinued, confirm continuous cessation period; if > 1 year the use must conform to the current district rules (verify with the City).
• If demolition occurred, measure exterior roof/wall area changed during any 24‑month window: > 50% (voluntary) or > 75% (involuntary) trigger full compliance with current Zoning Ordinance (involuntary exception for FAR may apply). Provide demolition documentation.
• Prepare a parking plan that meets § 20.60.090 (off‑street parking) or explain why no additional parking is required under the ordinance’s changed-use rule.
• If converting occupancy class or intensifying use (e.g., non‑habitable → habitable), confirm building code (Title 24) compliance and coordinate with building inspections (California Building Standards Code).
• Check whether Design Review is required for the proposed work; consult Mill Valley Design Review procedures and exemptions (see Section 20.66.030 / 20.66.045 references in the code).
• If the property sits inside an overlay (Historic, Housing Overlay, Floodplain, etc.), follow overlay rules and confirm exceptions (see Overlay Districts).
• Consult Planning staff early for parcel‑specific interpretation and any required Conditional Use Permit (e.g., to allow a commercial use in a residential zone for the remaining economic life).

Risks & Ambiguities

Plain‑English summary

Mill Valley's zoning code lets lawful nonconforming uses and buildings continue but restricts expansion, penalizes abandonment (one‑year rule), and triggers full compliance if you demolish enough of a nonconforming building (50% voluntary / 75% involuntary thresholds shown in the ordinance excerpts). Repairs and limited upgrades are allowed, but converting or intensifying occupancy will generally require bringing the building into conformity with zoning and building codes. Always verify parcel‑level rules with Planning because design review, overlays, parking, and ADU/state law interactions can change the outcome.

Source References

• Chapter 20.08 (Definitions) — nonconforming building/use definitions and related definition text.
• Ordinance excerpts containing abandonment / re‑establishment language (one‑year rule) and parking cross‑reference to § 20.60.090.
• Ordinance excerpts on replacement after demolition (voluntary 50% / involuntary 75% thresholds) and exceptions (seismic, Building Code, floodplain, voluntary upgrades).
• RM‑3.5 district permitted uses and base dimensional standards (multi‑family table and note that existing single‑family homes are not treated as nonconforming).
• Downtown Residential (DR) district development standards and notes on single‑family treatment.
• Design Review chapter and exemptions — references to § 20.66.030 and § 20.66.045 in design review guidance.
• Off‑street parking requirements referenced at § 20.60.090 (parking re‑establishment rules referenced in nonconforming text).
• Accessory Dwelling Unit (ADU) chapter references and state ADU law notes that local rules cannot always require the correction of nonconforming zoning conditions for ADU approval (see Chapter 20.90 and state law discussion).

(These citations are to material retrieved from the Mill Valley Title 20 Zoning text provided in your uploaded materials; see the file citations above for the exact excerpted text.)

Information Gaps

• Exact single § number(s) in Title 20 that carry the primary nonconforming‑uses chapter text (the retrieved excerpts show the demolition thresholds, abandonment language and definitions but the precise local section number(s) for that nonconforming chapter were not clearly labeled in the excerpts supplied). The text is present in the uploaded ordinance material, but the explicit section identifier for the nonconforming chapter was not visible in the retrieved snippets. Verify with the full Title 20 PDF or the City.
• Parcel‑specific application (whether your lot triggers design review limits, overlay provisions, or historic‑overlay exceptions) — Verify with Planning staff and property file.

Retrieved passages

• CBC § 4 (title for) High relevance show source text
• Mill Valley Zoning Code (§ 2) High relevance show source text
• CBC § 1 (§ 1) High relevance show source text
• Mill Valley Zoning Code (Section 20.66.045.) Medium relevance show source text
• CBC § 6 (Title 24) Medium relevance show source text
• Mill Valley Zoning Code (§ 7) Medium relevance show source text
• Mill Valley Zoning Code (Section 20.66.045.) Medium relevance show source text
• Mill Valley Zoning Code (title to) Medium relevance show source text
• Mill Valley Zoning Code (Chapter 20.16) Medium relevance show source text
• Mill Valley Zoning Code (Section 5401) Medium relevance show source text
• Mill Valley Zoning Code (§ 2) Medium relevance show source text
• Mill Valley Zoning Code (§ 66314) Medium relevance show source text
• Mill Valley Zoning Code Medium relevance show source text
• Mill Valley Zoning Code (§ 66333) Medium relevance show source text
• CBC § 66314 (§ 66314) Medium relevance show source text
• Mill Valley Zoning Code (chapter shall) Medium relevance show source text
• Mill Valley Zoning Code (Chapter 20.64) Medium relevance show source text
• Mill Valley Zoning Code (§ 2) Medium relevance show source text
• Mill Valley Zoning Code (Section 6409) Medium relevance show source text

Cited sections

• Chapter 20.08 (Definitions) — nonconforming building/use definitions and related definition text. (Chapter 20.08)
• Ordinance excerpts containing abandonment / re‑establishment language (one‑year rule) and parking cross‑reference to **§ 20.60.090**. (§ 20.60.090)
• Ordinance excerpts on replacement after demolition (voluntary 50% / involuntary 75% thresholds) and exceptions (seismic, Building Code, floodplain, voluntary upgrades).
• RM‑3.5 district permitted uses and base dimensional standards (multi‑family table and note that existing single‑family homes are not treated as nonconforming).
• Downtown Residential (DR) district development standards and notes on single‑family treatment.
• Design Review chapter and exemptions — references to **§ 20.66.030** and **§ 20.66.045** in design review guidance. (chapter and)
• Off‑street parking requirements referenced at **§ 20.60.090** (parking re‑establishment rules referenced in nonconforming text). (§ 20.60.090)
• Accessory Dwelling Unit (ADU) chapter references and state ADU law notes that local rules cannot always require the correction of nonconforming zoning conditions for ADU approval (see Chapter 20.90 and state law discussion). (chapter references)
• MillValley_ZoningCode.md
• 2025 California ADU handbook.md

• Official Mill Valley code library