California ADU rules · Los Angeles County
Can I Build an ADU in Downey?
Yes — you can build an ADU in Downey. California's statewide ADU law requires every city, including Downey, to approve a qualifying accessory dwelling unit by right, plus a junior ADU (JADU) on a single-family lot. What varies city to city are the local specifics below.
Can you build one?
ADUs allowed
CA state lawYes — by right
ADUs and JADUs are allowed by right in all residential and mixed-use zones; ministerial approval, no hearing.
Units on a single-family lot
Local ruleMax 2 ADUs (one attached or detached new-construction + one conversion), plus 1 JADU (converted from existing space)
Allows both a new-construction ADU and a conversion ADU, plus a JADU, on single-family lots.
Units on a multifamily lot
Local ruleUp to 8 detached ADUs (not to exceed number of existing units), plus up to 25% of existing units as conversion ADUs (minimum 1). No JADUs allowed.
No JADUs allowed on multifamily lots; detached ADUs capped at 8 or number of existing units.
Junior ADU (JADU)
Local ruleAllowed
JADUs allowed only on single-family lots, not multifamily.
Size & height
Max detached ADU size
Local rule1,000 sq ft
Detached ADUs: 850 sq ft (studio/1-bed), 1,000 sq ft (2+ bedrooms); state floor of 800 sq ft still applies.
Max attached ADU size
Local rule800 sq ft or 50% of the floor area of the existing primary dwelling, whichever is greater, but not to exceed 1,000 sq ft
Local cap is 1,000 sq ft for attached ADUs.
Guaranteed minimum size
CA state law800 sq ft
An 800 sq ft ADU must be allowed regardless of FAR/lot-coverage caps; a city also cannot cap an attached ADU below 850 sq ft (1,000 for 2+ bedrooms).
Max JADU size
CA state law500 sq ft
Max height
CA state law16 ft
16 ft baseline; cities must allow 18 ft within ½ mi of a major transit stop or where the primary dwelling is two-story, and 20 ft on a lot with a multistory multifamily building.
Setbacks
Side setback
CA state law4 ft
A city cannot require more than 4 ft side or rear setback; conversions of existing structures are exempt.
Rear setback
CA state law4 ft
Front setback
Local ruleADUs generally not allowed in front setback, except if physically infeasible to build an 800 sq ft ADU elsewhere on the lot (requires city review and documentation)
Front setback exception only if no feasible alternative for 800 sq ft ADU.
Separation from main house
Local ruleNone required
No specific separation required for new detached ADUs.
Parking
Parking required
Local ruleUp to 1 space
1 uncovered off-street space per ADU; multiple exemptions apply.
Transit parking exemption
CA state lawYes — no parking required near transit & for conversions
No parking may be required within ½ mi walking distance of transit, in a historic district, for an ADU within an existing structure, within one block of car-share, or in certain permit-parking areas.
Garage-conversion replacement parking
Local ruleNot required
No replacement parking required for garage/carport conversion.
Approval, timeline & cost
Approval path
CA state lawMinisterial (by right)
No public hearing or discretionary design review for a qualifying ADU.
Approval timeline
CA state law60 days
The agency must act within 60 days of a complete application — and the ADU is deemed approved if it does not.
Owner-occupancy
Local ruleOnly for a JADU
Owner-occupancy required for JADUs only; not for ADUs.
Impact-fee waiver
CA state lawWaived under 750 sq ft
An ADU of 750 sq ft or less pays no impact fees (the 750 sq ft threshold itself is exempt under § 66311.5); larger ADUs pay fees proportional to the primary dwelling. JADUs (≤ 500 sq ft) are exempt.
Permits required
Local ruleBuilding permit, Demolition permit (for garage conversion, issued concurrently)
Demolition permit required for garage conversion, issued with building permit.
Downey-specific factors that can change the answer
State law sets the floor, but these local conditions often decide whether a specific parcel works.
Sprinkler requirements
Sprinklers required in ADU only if required in primary dwelling.
Downey limits detached ADUs to 1,000 sq ft (2+ bedrooms) and attached ADUs to 1,000 sq ft, with nuanced height rules and a cap of 8 detached ADUs on multifamily lots. JADUs are allowed only on single-family lots, and no replacement parking is required for garage conversions.
Frequently asked questions
Can I build an ADU in Downey?
Yes. California's statewide ADU law requires Downey to approve a qualifying accessory dwelling unit ministerial (by right), plus a junior ADU (JADU) on a single-family lot.
How big can an ADU be in Downey?
A detached ADU of at least 1,000 sq ft must be allowed, and an ADU of 800 sq ft can never be zoned out by floor-area or lot-coverage limits. A JADU can be up to 500 sq ft.
What are the setback requirements for an ADU in Downey?
Side and rear setbacks are limited to 4 ft. ADUs generally not allowed in front setback, except if physically infeasible to build an 800 sq ft ADU elsewhere on the lot (requires city review and documentation).
Is parking required for an ADU in Downey?
Up to 1 space. No parking can be required within ½ mile of transit, for conversions, or for units inside the primary home. When a garage is converted, replacement parking is not required.
How long does ADU approval take in Downey?
A complete application must be acted on within 60 days, with ministerial (by right) — no public hearing or discretionary design review for a qualifying ADU.
Do I have to live on the property to build an ADU in Downey?
Owner-occupancy: only for a jadu. ADUs under 750 sq ft are also exempt from impact fees.
Sources
- Downey Municipal Code, Article IX, Section 9414 (Ord. 25-1523, effective 4/8/2025)
- §9414.08(a)
- §9414.10(a)
- §9414.08(a)(2)(i)
- §9414.08(b)(1), (6); §9414.10(b)(1), (5)
- §9414.08(b)(2)
- §9414.08(c), §9414.10(c)
- §9414.08(c)(3)
- §9414.14(a)
- §9414.08(a)(2)(vii)
- §9414.16(d)
- §9414.14(b)
- Cal. Gov. Code §§ 66310–66342 (ADU Law)
- HCD 2025 Accessory Dwelling Unit Handbook
- Cal. Gov. Code § 66317 (ministerial approval, 60-day deadline)
- Cal. Gov. Code § 66321 (size minimums & height)
- Cal. Gov. Code § 66333 (junior ADUs)
- Cal. Gov. Code § 66323 (by-right ADU/JADU, 800 sq ft, 4-ft setbacks)
- Cal. Gov. Code § 66322 (ADU parking; § 66334 covers JADU parking)
- Cal. Gov. Code § 66311.5 (impact fees: none for an ADU ≤ 750 sq ft or JADU ≤ 500 sq ft)
Last updated April 8, 2025. This is an AI-assisted summary of Downey's ADU rules, drawn from its published ordinance and California ADU law — general information, not legal advice, and it may be incomplete or out of date. Verify the controlling rule for your specific parcel with Downey Planning before relying on it.
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