One State Framework, Nine Counties.
Thanks to a decade of California housing laws, ADUs are now allowed on virtually any residential or mixed-use lot with an existing or proposed home, anywhere in the nine Bay Area counties. Every city and county must review a complete application within 60 days, most permits clear in three to four months, and state law requires approval of qualifying ADUs even where local rules would otherwise block them.
The result: ADUs now make up 13.4% of the Bay Area's building permits. But the details still shift city by city, which is why the fastest first step is to check your specific address rather than reason from a neighbor's project two towns over.
Size And Height Limits Vary By City.
The regional pattern: many jurisdictions, including San Jose, Oakland, Alameda County, and Redwood City, allow detached ADUs up to 1,200 square feet, while others cap units at 850 or 1,000 square feet depending on lot size or bedroom count. Every city must allow at least 16 feet of height, rising to 18 to 20 feet near transit or on multifamily lots when the roof pitch matches the main home.
Attached ADUs can also reach 1,200 square feet, though many cities limit them to 50% of the main home's floor area. State law overrides that ratio whenever it would prevent an ADU of at least 850 to 1,000 square feet. Junior ADUs stay within the existing home's footprint at 500 square feet max, and the minimum size for any ADU is 150 square feet.
Setbacks, Overlays, And Design Rules.
Setbacks are one of the few constants: 4 feet from side and rear property lines everywhere, with detached units typically needing 6 feet of separation from other structures. Converting existing space or rebuilding on the same footprint skips setbacks entirely, and flood zones can add height allowances for elevation.
Fire, flood, and coastal overlay areas still allow ADUs, just with extra safety or design requirements, and historic districts permit them too. Where cities impose design standards, they must be objective: matching rooflines, materials, and colors, especially street-facing. Most Bay Area ADUs also require no dedicated parking, with exemptions near transit, in historic districts, near car-share, and for garage conversions.
How Many Units You Can Add.
On a single-family lot anywhere in the region, you can build one detached or attached ADU plus one JADU inside the existing home. On multifamily lots, state law allows up to eight detached ADUs (never more than the existing unit count) plus interior conversions of up to 25% of existing units; Abodu's multifamily program is built for exactly that.
And the rules keep moving in homeowners' favor: recent legislation like SB 897 and SB 1033 has continued to expand heights, sizes, and approval rights across the state.
What It Actually Costs.
Custom site-built ADUs in California routinely exceed $250,000 before change orders, and Bay Area labor and site conditions push that higher still. Abodu is transparent published pricing: homes from $234,800 plus a published installation price, an expected all-in from $298,800, covering design, permits, the factory build, delivery, and installation, the same number in San Jose as in Marin. See how the process works, then check your lot to get a city-specific answer.