Local zoning · Atherton

Atherton — Nonconforming Uses

Q: What happens to a nonconforming use if I stop operating it for a few months?

If a nonconforming use is abandoned for six continuous months or more , the right to continue that nonconforming use terminates under Town code (nonconforming status ends at six months). Keep records (leases, bills) to prove continuous use if needed. § 17.26.030(C) .

Q: If I fix up a nonconforming accessory building, do I need to meet current setbacks?

Maintenance and repairs that do not increase the degree of nonconformity are allowed; alterations to accessory buildings may be permitted if done to the conforming portion or to make the nonconforming portion conform. If work increases nonconformity or exceeds thresholds, current standards apply. § 17.26.040(B)-(C)(3) .

Q: Does an ADU application get blocked by unrelated nonconforming zoning issues on my lot?

No — Atherton’s ADU chapter specifically provides that nonconforming zoning conditions need not be corrected as part of an ADU approval provided they do not present a threat to public health or safety and are not affected by the ADU construction. Chapter 17.52 .

Q: My lot is in POS — can a building on it keep a nonconforming use?

Yes — nonconforming uses/structures in the POS district are treated under the same nonconforming chapter; but permitted uses in POS are limited and some new uses require a CUP. Check § 17.34.030 and the nonconforming rules. § 17.34.030; § 17.26.030 .

Q: If I do multiple small projects over time, when does the 50% rule apply?

The code counts work cumulatively over a five‑year period : if more than 50% of existing floor area or of nonconforming exterior walls is rebuilt either in a single project or cumulatively over five years, the building must be brought into full compliance with current zoning and building rules. § 17.26.040(C)(2) .

Q: Who approves exceptions for additions that would extend a nonconforming building in R‑1B?

Additions beyond the main building area in R‑1B require a Special Structures Permit and are reviewed by the Planning Commission; the permit process and findings are in Chapter 17.15. § 17.26.040(E); § 17.15.020–.040 .

Q: Where can I check my lot’s zoning, overlays, and whether a nonconforming rule applies?

Start with the Town’s zoning maps and the 17.30 district designations; confirm overlays (RM‑10, RM‑20/40, Parker Ave) that can change setbacks and allowable uses. Verify findings with the Town planner; interpretation of district boundaries is entrusted to the Town planner. § 17.30.010; § 17.30.060 . ---

Nonconforming Uses under the Atherton local zoning and planning code, with the controlling citations.

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Last reviewed: July 3, 2026

Overview

This page explains how the Town of Atherton treats nonconforming uses, nonconforming structures, and nonconforming lots under the Town’s zoning code (Title 17). It summarizes who can continue a pre‑existing use/structure, what changes are allowed, and the special rules that apply in each local district (the R-1A, R-1B, POS, and PFS districts). All requirements below cite the controlling code sections; verify parcel‑specific questions with the Town. § references and the underlying ordinance text are cited from the Town’s Title 17 materials.

What the code says (key rules)

Continuation: A lawful use or building that becomes nonconforming may continue in its current form. § 17.26.030(A).
Restrictions on nonconforming uses (no expansion/relocation/change): Nonconforming uses shall not be enlarged, changed to another nonconforming use, extended to occupy greater land area, or moved to another portion of the lot. § 17.26.030(B).
Abandonment/termination: If a nonconforming use is abandoned for six continuous months or more, the nonconforming rights terminate. § 17.26.030(C).
Nonconforming structures: Buildings lawful when built but now nonconforming may be continued, maintained, and repaired so long as repairs do not increase the degree of nonconformity. § 17.26.040(A)-(B).
Repair / reconstruction after damage: A legal nonconforming main building damaged by accidental occurrence may be restored within the prior building envelope, but reconstruction must meet current building standards. § 17.26.040(C)(1).
Threshold for larger alterations/additions: Additions or reconstructions that rebuild more than 50% of existing floor area or more than 50% of nonconforming exterior walls (single project or cumulative ≤5 years) must comply with current zoning and building code requirements. Interior-only work is excepted. § 17.26.040(C)(2).
Solar: Adding solar facilities is expressly not treated as increasing nonconforming standards. § 17.26.040(C)(4).
District-specific special rules (examples below) modify the general framework for residential districts. See each district subsection for the controlling provisions.

Note: accessory-dwelling‑unit (ADU) processing includes a local rule that nonconforming conditions need not be corrected as part of an ADU approval where the condition does not threaten health/safety and isn't affected by the ADU. § 17.52 (ADU chapter).

District‑by‑district breakdown

Below each district entry you’ll find: purpose, typical permitted uses (short), key dimensional/development standards that matter for nonconformity decisions, and where the district applies.

R-1A (Residential district R-1A)

Purpose: Implements single‑family residential general plan categories for the town. § 17.32.010.
Typical permitted uses: single‑family dwelling, home occupations, employee housing (small), second dwelling units (ADUs) (subject to Chapter 17.52). See Table 17.32.030-1. § 17.32.030.
Key dimensional standards (decision‑relevant): standard main building max height 30 ft (vertical sidewalls limited to 22 ft; with special-structure permit height can be increased up to 34 ft in limited circumstances), front yard 30 ft minimum (see exceptions), and lot coverage and side/rear rules in Chapters 17.32/17.35/17.38. § 17.32.040.
How nonconforming rules interact: Additions to a legal nonconforming main building in R-1A may maintain existing setback lines so long as the addition does not encroach more than 20% into the current R‑1A setbacks and no portion is taller than 30 ft (specific allowance in § 17.26.040(D)). § 17.26.040(D).
Where it applies: Most single‑family residential parcels; see the Atherton zoning plan maps. § 17.30.010.

Relevant local pages: see the Town’s Atherton Development Standards and the Atherton ADUs page for ADU processing specifics (linked where first referenced in this document).

R-1B (Residential district R-1B)

Purpose: Similar residential intent to R-1A but with different height/yard formulas and typical lot sizes. § 17.33.010–.040.
Typical permitted uses: single‑family dwellings, home occupations, second dwelling units (ADUs), limited public utilities and community uses (see Table 17.33.030-1). § 17.33.030.
Key dimensional standards (decision‑relevant): main building max height 28 ft, vertical sidewalls/columns may not exceed 18 ft, and other setback/lot coverage rules in 17.33.040. § 17.33.040.
How nonconforming rules interact: Additions to a legal nonconforming main building in R-1B are permitted only within the main building area; proposed additions beyond the main building area may be allowed only with a Special Structures Permit (Planning Commission) per § 17.26.040(E)(1)(a) and chapter 17.15. § 17.26.040(E); § 17.15.020(F).
Where it applies: Neighborhoods mapped as R‑1B on the Atherton zoning maps. § 17.30.010.

Practical note: If a homeowner in R-1B wants an addition that extends outside the original main building area, expect a quasi‑judicial review (special structures permit) and findings about privacy/compatibility. § 17.15.040.

POS (Park and Open Space district)

Purpose: Preserve natural resources and provide parks/recreation/open space, consistent with the General Plan and California open‑space code. § 17.34.010.
Typical permitted uses: public parks, watershed/groundwater recharge, country clubs (CUP), caretaker housing (CUP), certain residential uses only where overlay permits (see Table 17.34.030-1). § 17.34.030.
Key dimensional standards: Chapter 17.34 development standards apply and conventional yard/height rules may be modified to preserve open space character; see Table 17.34.040 and the chapter. § 17.34.040.
How nonconforming rules interact: Nonconforming uses/structures in POS follow the same continuation/abandonment/repair principles in Chapter 17.26; permitted use changes still require CUP where the POS table lists CUP. § 17.26.030; § 17.34.030.

PFS (Public Facilities and School district)

Purpose: Accommodates public facilities, schools, and associated uses. § 17.36.010–.040.
Typical permitted uses: public schools, municipal buildings, recreation associated with public facilities; some uses require CUP. Table 17.36.040 lists permitted uses and standards. § 17.36.040.
Key dimensional standards (decision‑relevant): tableed standards include maximum height 34 ft, lot coverage 40%, minimum front yard 60 ft, and larger side/rear setbacks where adjacent to residential parcels (50–75 ft depending on building height). See Table 17.36.040. § 17.36.040.
How nonconforming rules interact: Nonconforming PFS uses and structures are subject to the general nonconforming rules in Chapter 17.26; because PFS often borders R‑1 properties, R‑1 adjacent setback/height interactions in Chapter 17.36 are especially material to additions/repairs that might change nonconformity. § 17.26.040; § 17.36.040.

Quick decision table

This table extracts the most decision‑relevant nonconforming standards and the exact code reference.

Rule / Permit outcome	What the code allows / prohibits	Code reference
Continue lawful pre‑existing use	May continue (no forced removal solely because of zoning change)	§ 17.26.030(A)
Expand a nonconforming use	Not allowed (no enlargement/change/relocation/area increase)	§ 17.26.030(B)
Abandonment	Nonconforming rights end after 6 months of continuous abandonment	§ 17.26.030(C)
Repairs & maintenance	Allowed if they do not increase degree of nonconformity	§ 17.26.040(B)
Major rebuild/addition threshold	>50% of floor area or nonconforming exterior walls (single project or cumulative ≤5 years) triggers compliance with current code	§ 17.26.040(C)(2)
R‑1A special addition rule	May keep existing setback lines if encroachment ≤20% and height ≤30 ft	§ 17.26.040(D)
R‑1B additions outside main building area	Require Special Structures Permit (Planning Commission)	§ 17.26.040(E); § 17.15.020(F)
ADU and nonconformance	ADU approvals cannot be denied solely because of unrelated nonconforming zoning conditions (if not a safety threat)	Chapter 17.52 (ADUs); see § 17.52.040–.050.

Checklist (what an applicant must demonstrate / submit when proposing work on a nonconforming building/use)

Identify the specific nonconforming element (use, structure, lot dimension) and cite the date the use/structure became lawful. § 17.26.010–.020.
Demonstrate that proposed repairs/alterations do not increase the degree of nonconformity; if they do, show compliance path with current code or that the work is ≤50% rebuild over 5 years. § 17.26.040(B)-(C).
If the project is in R-1B and extends beyond the main building area, prepare a Special Structures Permit application addressing the Chapter 17.15 findings. § 17.26.040(E); § 17.15.040.
If the project is in R-1A and would encroach into setbacks, document the amount of encroachment (must be ≤20% and height ≤30 ft for the special allowance). § 17.26.040(D).
For reconstruction after damage, provide proof of the prior building envelope (surveys, historic plans) and meet current building standards. § 17.26.040(C)(1).
If proposing an ADU, follow Chapter 17.52 submittal rules and note that unrelated nonconforming issues may not block ADU approval (if no safety threat). § 17.52.
Provide accurate plans showing setbacks, height calculations (per Chapter 17.42), and certify any measurements required by staff. § 17.42; § 17.54.

For related technical checks see the Town’s pages on Atherton Zoning, Atherton Development Standards, and Atherton ADUs (linked on first natural mention above).

Risks & Ambiguities

Issue	Why it matters	What to verify
Abandonment timing (6 months)	If use stops for ≥6 months, nonconforming rights are lost (affects ability to resume use). § 17.26.030(C).	Verify continuous use records (leases, utility bills, permits) and discuss with Town planner.
Cumulative repairs/additions that hit the 50% trigger	Multiple smaller projects over a 5‑year window can convert a nonconforming structure into a fully conforming requirement (i.e., must meet current code). § 17.26.040(C)(2).	Calculate cumulative floor area replaced; obtain town confirmation before permitting.
Parcel‑specific overlay rules (RM overlays, Parker Ave)	Overlays or adjacent‑zone rules can change setbacks, allowable floor area, or permit paths—affects whether an addition is allowed. See overlay chapters and Parker Avenue rules. § 17.30.080; § 17.37.	Check the parcel’s overlay map and any special district notes on the zoning plan. Verify with Town planner.
ADU vs nonconforming structures	State ADU law limits local denial grounds; local ADU chapter clarifies that nonconforming zoning conditions may not block ADUs in many cases. § 17.52.	For ADU proposals, confirm which nonconforming items are unrelated and whether a safety issue exists.
Discrepancies in “building envelope” proof after accidental damage	Reconstruction within prior envelope is allowed, but burden of proof is on applicant. § 17.26.040(C)(1).	Prepare historic site plans, surveys, photos; expect Town to require clear evidence.
Whether a proposed change “increases degree of nonconformity”	That phrase is open to interpretation (e.g., small projection vs. structural rebuild). § 17.26.040(B)-(C).	Early pre‑application meeting with Town planner recommended; document scope and calculations.

Plain‑English summary

If your building or use was legal when it was created, Atherton lets it keep existing operations, but you cannot expand or move it, and if you stop using it for six months the legal nonconforming status is lost; modest repairs are allowed but major rebuilds or cumulative work over certain thresholds must meet current zoning rules. § 17.26.030–.040.

Source References

Title 17 (Zoning) — Chapter 17.26 Nonconforming Uses, Buildings, and Structures: § 17.26.010–.040.
Chapter 17.32 Residential District R‑1A (purpose, uses, standards): § 17.32.010–.040.
Chapter 17.33 Residential District R‑1B (allowed uses; development standards): § 17.33.030–.040.
Chapter 17.34 Park and Open Space (POS): § 17.34.010–.040.
Chapter 17.36 Public Facilities & Schools (PFS) (development standards table): § 17.36.040.
Chapter 17.15 Special Structures Permit (authority & findings; R‑1B additions): § 17.15.020–.050.
Chapter 17.52 Accessory Dwelling Units (ADU rules and nonconformance guidance): § 17.52.040–.050.
Definitions (nonconforming building/structure/use): Chapter 17.60.

Sources

Retrieved passages

CBC § 1 (§ 1) High relevance
Atherton Zoning Code (title shall) High relevance
Atherton Zoning Code (Title 17) High relevance
Atherton Zoning Code (title or) High relevance
Atherton Zoning Code (§ 1) Medium relevance
CBC § 1 (§ 1) Medium relevance
Atherton Zoning Code (§ 1) Medium relevance
Atherton Zoning Code (Title 17) Medium relevance
Atherton Zoning Code (section 17.32.040) Medium relevance
Atherton Zoning Code (Title 17) Medium relevance
CBC § 66314 (§ 66314) Medium relevance
Atherton Zoning Code (chapter 17.42) Medium relevance
Atherton Zoning Code (§ 1) Medium relevance
Atherton Zoning Code (section 17.36.060) Medium relevance
Atherton Zoning Code (chapter is) Medium relevance
Atherton Zoning Code (§ 1) Medium relevance
Atherton Zoning Code (section 17.52.050) Medium relevance

Cited sections

Official Atherton code library

Frequently asked questions

What happens to a nonconforming use if I stop operating it for a few months?

If a nonconforming use is abandoned for six continuous months or more, the right to continue that nonconforming use terminates under Town code (nonconforming status ends at six months). Keep records (leases, bills) to prove continuous use if needed. § 17.26.030(C).

Can I add a small addition to a nonconforming house in R‑1A?

Yes, but additions to a legal nonconforming main building in R‑1A may only keep the existing setbacks if the addition does not encroach more than 20% into current R‑1A setbacks and no portion exceeds 30 ft in height (special allowance). Larger work may trigger full compliance with current development standards. § 17.26.040(D).

Can I rebuild a nonconforming house after a fire?

A legal nonconforming main building damaged by accidental occurrence may be restored within the prior building envelope, but reconstruction must meet current building standards; the applicant bears the burden of proving the previous envelope. § 17.26.040(C)(1).

If I fix up a nonconforming accessory building, do I need to meet current setbacks?

Maintenance and repairs that do not increase the degree of nonconformity are allowed; alterations to accessory buildings may be permitted if done to the conforming portion or to make the nonconforming portion conform. If work increases nonconformity or exceeds thresholds, current standards apply. § 17.26.040(B)-(C)(3).

Does an ADU application get blocked by unrelated nonconforming zoning issues on my lot?

No — Atherton’s ADU chapter specifically provides that nonconforming zoning conditions need not be corrected as part of an ADU approval provided they do not present a threat to public health or safety and are not affected by the ADU construction. Chapter 17.52.

Can a nonconforming use be changed to a different use?

Not to another nonconforming use. The code prohibits changing a lawful nonconforming use to any other nonconforming use; changes must comply with the district’s allowed uses and permit paths. § 17.26.030(B).

My lot is in POS — can a building on it keep a nonconforming use?

Yes — nonconforming uses/structures in the POS district are treated under the same nonconforming chapter; but permitted uses in POS are limited and some new uses require a CUP. Check § 17.34.030 and the nonconforming rules. § 17.34.030; § 17.26.030.

If I do multiple small projects over time, when does the 50% rule apply?

The code counts work cumulatively over a five‑year period: if more than 50% of existing floor area or of nonconforming exterior walls is rebuilt either in a single project or cumulatively over five years, the building must be brought into full compliance with current zoning and building rules. § 17.26.040(C)(2).

Who approves exceptions for additions that would extend a nonconforming building in R‑1B?

Additions beyond the main building area in R‑1B require a Special Structures Permit and are reviewed by the Planning Commission; the permit process and findings are in Chapter 17.15. § 17.26.040(E); § 17.15.020–.040.

Where can I check my lot’s zoning, overlays, and whether a nonconforming rule applies?

Start with the Town’s zoning maps and the 17.30 district designations; confirm overlays (RM‑10, RM‑20/40, Parker Ave) that can change setbacks and allowable uses. Verify findings with the Town planner; interpretation of district boundaries is entrusted to the Town planner. § 17.30.010; § 17.30.060. ---

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