Rules by city
ADU rules in Unincorporated LA County
The key facts first, then what we've learned building there.
Key facts
The snapshot
Checked October 2026. We re-check every lot against the current code.
LA County (unincorporated)
| Rule | Answer |
|---|---|
| Detached ADU size | Up to 1,200 sq ft |
| Setbacks | 4 ft side and rear |
| Height | 16 ft; more in some cases |
| Pre-approved plans | Yes: free county Plans A, B and C, built as drawn (mirroring allowed) |
| Fire rebuild lane | Yes, after the 2025 fires |
| SB 1211 apartment ADUs | Yes |
| Building department | LA County Public Works (permits) and Regional Planning (zoning) |
Local notes
What's different here
01
Which rules apply
Areas like Altadena, East LA and Topanga are unincorporated, so the county's rules apply, not a city's.
02
Fire rebuild
The county runs an expedited lane for homes lost in the 2025 fires; an ADU can be part of it.
Programs
What applies here
SB 1211 Multifamily ADUs
Since January 2025, California lets an existing apartment property add detached ADUs, one for every existing unit up to eight, plus conversions of garages and storage.
Learn more
SB 9 Lot Split
SB 9 lets a single-family lot be split in two, with up to two homes on each.
Learn more
Pre-Approved Plans
Cities must now keep a set of pre-approved ADU plans.
Learn moreFloor plans
Plans that fit
Ready-drawn plans we adapt to your lot and your city.
Ask about your address
We check the lot against these rules and tell you what fits.