You sketch a 1,000-square-foot backyard cottage, take it to the counter, and the planner says the city's ADU cap is smaller. A neighbor heard you also have to live on-site. Someone else mentions two parking spaces. Here's the thing about ADU rules in California: most of the limits you'll be quoted are themselves limited by state law — and where a city ordinance goes past what the state allows, the state wins.
The framework is the state ADU law (Government Code §§ 66310–66342). Under § 66314, an attached ADU may not exceed 50% of the existing home's floor area, a detached ADU may not exceed 1,200 square feet, side and rear setbacks are capped at 4 feet, and parking is capped at one space per unit or bedroom. And separately, every city must approve at least one detached new-construction ADU of up to 800 square feet meeting the height limits — the guaranteed tier the state enforces by name.
The numbers, and which way they cut
A crucial reading habit: most of these figures are ceilings on the city's power, not requirements on your project.
| Standard | The state rule | Where it says so |
|---|---|---|
| Detached ADU size | Total floor area not to exceed 1,200 sq ft | § 66314(d)(5) |
| Attached ADU size | Not to exceed 50% of the existing primary dwelling | § 66314(d)(4) |
| Guaranteed approval | One detached new-construction ADU up to 800 sq ft (meeting height limits) must be approved | HCD, citing § 66323(a)(2) in its Glendale enforcement letter |
| Side/rear setbacks | City may require no more than 4 ft; none for qualifying conversions | § 66314(d)(7) |
| Parking | ≤1 space per unit or bedroom (whichever is less); tandem on a driveway allowed | § 66314(d)(10) |
| Parking near transit | Zero may be required within a half-mile walk of transit (and 5 other cases) | § 66322 |
| Owner-occupancy | May not be required (30-day minimum rentals may be) | § 66315 |
The setback provision is the cleanest example of the direction rule, so it's worth reading in the law's own words:
"a setback of no more than four feet from the side and rear lot lines shall be required" — § 66314(d)(7)
That binds the city — it may not demand more than four feet. It does not mean your ADU needs four feet from the fence in every case: qualifying garage conversions and same-footprint rebuilds get no setback requirement at all under the same paragraph.
"Maximum standards": why the city can't pile on extras
§ 66315 declares § 66314 the maximum set of standards for an ADU on a single-family lot — "no additional standards" may be imposed, with owner-occupancy called out as expressly off-limits. And § 66319 blocks the classic workaround: an ADU can't be counted against the lot's density or growth limits.
This isn't theoretical. The corpus includes the state's 2025 Notice of Violation to Glendale, where HCD calls the city's attempt to block detached ADUs on lots that already have a pool house or guest quarters "a clear violation" of the guaranteed-approval rule — "State law does not contain a carveout for lots with other detached accessory structures." If your city quotes a rule that contradicts the table above, the polite phrase to use at the counter is: "Can you show me where state ADU law authorizes that standard?"
How the square footage is measured
Size fights are often measurement fights. Per the state's ADU Handbook, floor area follows the building code's "gross floor area" definition — the area inside the exterior walls, counting closets and interior walls, excluding vent shafts and courts — unless your city has formally amended the definition locally. A porch or an area without surrounding walls counts differently, which is exactly the kind of detail that decides whether a design is 790 or 810 square feet.
What this means for your project
Design to the tier you need. If the guaranteed 800-square-foot detached unit covers your plans, your approval path is the strongest state law offers. Between 800 and 1,200 square feet, you're in territory the city may regulate (height, size caps above the floor, architectural standards) but only with objective standards under § 66314(b). State law also sets minimum sizes cities must allow for smaller units — and every city fills in its own details on top, so check your city's specific rules on our city-by-city ADU comparison before finalizing a design.
Sources
- Government Code § 66314 — ADU ordinance standards (size, setbacks, parking)
- Government Code § 66315 — Maximum standards; no owner-occupancy requirement
- Government Code § 66319 — ADUs and density limits
- Government Code § 66322 — Parking exemptions
- HCD Notice of Violation — Glendale ADU ordinance (Aug 2025)
- 2025 California ADU Handbook — Size requirements
