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Stanislaus County, CA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified August 2026

ADU rules in Stanislaus County, CA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Max ADU size: 1,200 square feet of living space
Max junior ADU size: 500 square feet
Single-family lot limit: 1 ADU plus 1 junior ADU
A-2 parcel exception: 20+ acres allows ADU in lieu of second unit
Zones permitted: R-1, R-2, R-3, R-A, MU, A-2, PD

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in California under recent ADU streamlining laws. The Stanislaus County, CA rules below cover local size, height, and setback requirements.

Summary

Stanislaus County allows one accessory dwelling unit plus one junior ADU on any single-family parcel in the unincorporated area, capping new ADUs at 1,200 square feet of living space and junior ADUs at 500 square feet under County Code § 21.74.030. Multi-family and duplex parcels get a separate, larger allowance.

City-specific rules exist: Modesto has its own adu rules rules that differ from Stanislaus County's county-level regulations. If you live in Modesto, check the city-specific page instead.

The total number of accessory dwellings permitted per parcel with only single-family dwelling(s), existing or proposed, as permitted by the applicable zoning district, shall be limited to the following: a. One junior accessory dwelling when located within the space of or attached to a proposed or existing single-family dwelling; and b. One detached or attached accessory dwelling... B. Accessory dwellings shall meet the following criteria: 1. The maximum square footage of a new accessory dwelling shall not exceed one thousand two hundred square feet of living space, or five hundred square feet of living space for a junior accessory dwelling, except for temporary mobile homes being converted to permanent status in accordance with Chapter 21.72 of this title... 2. The accessory dwelling shall have a separate entrance and no shared living space, or connecting interior access, with the main dwelling, unless the unit meets the definition of a junior accessory dwelling.

View official code

Full Breakdown

Chapter 21.74 lets accessory dwelling units and junior ADUs go in every residential zoning district in the unincorporated county, R-1, R-2, R-3, R-A, Mixed-Use and A-2 General Agriculture, plus any Planned Development district that allows housing (§ 21.74.020). On a single-family lot the limit is one attached or detached ADU plus one junior ADU built inside the existing house; a duplex parcel can add one junior ADU and two detached ADUs, and an existing multifamily building can convert up to 25 percent of its non-livable floor area (storage rooms, boiler rooms, basements, garages) into attached ADUs, or add up to eight detached ADUs (§ 21.74.030(A)).

Size is capped at 1,200 square feet of living space for a standard ADU and 500 square feet for a junior ADU, and the unit must have its own entrance with no shared living space with the main house unless it qualifies as a junior ADU (§ 21.74.030(B)). On A-2 agricultural parcels of 20 or more acres, one ADU is allowed in lieu of a second dwelling, and any detached unit must sit within 150 feet of the main house and avoid displacing land in crop production unless the Planning Director grants a staff-approval exception (§ 21.74.030(A)(4)(c), (C)(1)).

Parcels inside a city's LAFCO-adopted sphere of influence follow that city's ADU standards instead of the county's, as long as they remain consistent with state law (§ 21.74.020(A)). Every ADU must also meet applicable water supply, wastewater and Fire Code standards, including stricter building requirements in designated high and very-high fire hazard severity zones (§ 21.74.030(E)-(G)).

Violations & Fines

Building an ADU beyond these unit or size limits, or in a zoning district not listed in § 21.74.020, is an unpermitted structure that the county's Building Permits Division and Code Enforcement can red-tag, require to be removed or downsized, and pursue through Stanislaus County's standard code-enforcement abatement process, with the property owner liable for compliance costs.

Frequently Asked Questions

How big can an ADU be on unincorporated Stanislaus County property?

Up to 1,200 square feet of living space for a standard accessory dwelling unit, or 500 square feet for a junior ADU built within an existing single-family home, per County Code § 21.74.030(B)(1). Mobile homes converted to permanent ADU status under Chapter 21.72 are the only exception to that cap.

How many ADUs can I build on a single-family lot in the county?

One detached or attached ADU plus one junior ADU inside the existing house, per § 21.74.030(A)(1). Duplex parcels get one junior ADU plus two detached ADUs, and existing multifamily buildings can add up to eight detached ADUs or convert 25 percent of non-livable floor area to attached units.

Do county ADU rules apply inside Modesto or Turlock?

No. Chapter 21.74 governs only the unincorporated area of Stanislaus County. Parcels within a city's LAFCO-adopted sphere of influence instead follow that city's own ADU ordinance, and incorporated cities like Modesto, Turlock and Ceres administer their own separate ADU codes for property inside city limits.

Sources & Official References

[1]Stanislaus County Code, Chapter 21.74 — Accessory Dwellings (eCode360)

Other rules in Stanislaus County

Parking Rules: Overnight ParkingSome Restrictions Parking Rules: Street Parking LimitsSome Restrictions Shed & Carport RulesSome Restrictions ADU Rental RestrictionsSignificant Restrictions Garage ConversionsFew Restrictions Tiny HomesSome Restrictions Noise Ordinances: Vehicle NoiseSome Restrictions Parking Rules: Abandoned VehiclesSignificant Restrictions Parking Rules: Loading ZonesSome Restrictions Parking Rules: Commercial & Oversized VehiclesSignificant Restrictions

All Stanislaus County rules

California rules heatmap·Compare Stanislaus County to another location·View the California accessory structures overview

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ADU Rules in Cities Across Stanislaus County

Modesto, CA