These are state-mandated minimums — the smallest, shortest ADU your city is legally required to allow, sourced from California Government Code § 66321 and the state HCD ADU Handbook. Your city may allow a larger unit than shown here, and your specific lot may have additional constraints (easements, existing structures, environmental overlays). This isn't legal advice — confirm your property's actual zoning and buildable envelope with your city's planning department. See our guide on finding your property's zoning to get started.
How to read these numbers
California ADU law sets a floor, not a ceiling. Every city in the state must allow at least the size, height and setback shown above for the ADU type you selected — they're free to allow more (and many do), but they can't require less. That's why this calculator can't tell you your property's actual maximum: only your city's zoning code and your lot's specific conditions can do that.
The one universal baseline underneath all of this: regardless of any other local size cap, lot coverage rule, floor-area ratio, or front-yard setback requirement, every city must allow at least an 800 sq ft ADU with just 4-foot side and rear setbacks. Everything above that — the 850/1,000 sq ft size floors, the height tiers — builds on top of that baseline.
Why your city might allow more than the state minimum
Many California cities set their local ADU standards well above the state floor, especially for detached units — it's common to see local maximums of 1,200 sq ft or more, particularly on larger lots. Some cities also apply a cap tied to your primary home's size (commonly 50% of the existing home's floor area for an attached ADU), which is allowed as long as it never pushes the minimum below the state floor for your unit type.
Height works similarly: the 18-foot tier applies to detached ADUs within a half-mile walk of a major transit stop or on a lot with an existing or proposed multifamily building — not just transit proximity. And nearly every tier allows an additional 2 feet if your ADU's roof pitch matches your primary home's, which is a detail worth raising with your architect or contractor early in design.
Garage and accessory-structure conversions work differently from new construction: because you're reusing a structure that's already there, state law exempts the conversion from size, height and lot-coverage standards entirely — your existing footprint is your ADU's footprint. The only added allowance is up to 150 sq ft beyond that footprint, and only if it's needed for required entry or exit, like a stairway to a second story.
This calculator shows the floor, not your ceiling
Use it to sanity-check whether a project idea is even in the realm of possibility before you invest time in design — not as a final answer on what you're allowed to build. A free property review and a conversation with your local building department are the next real steps.
Frequently asked questions
The minimum. These are the state-mandated floors every California city must allow regardless of local zoning — your specific city's actual maximum may be larger. Check your local zoning code or ask your planning department for your property's real limit.
No — these state-law floors apply regardless of lot size. Lot size can still matter for other things, like whether there's physically enough space to meet setbacks, or whether your city's local (higher) size allowance scales with lot size.
Correct — state law exempts conversions of existing legal accessory structures from size, height and lot-coverage standards. Your existing structure's footprint becomes your ADU's footprint, plus up to 150 sq ft more if you specifically need it for a required entry or exit. See our garage conversion vs. detached ADU guide for how the two paths compare on cost and timeline.
Your city's planning or building department, usually through an online zoning lookup tool. See our guide to finding your property's zoning for how to do that, and check our service area guide if you're in one of the cities we currently cover.
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