- State-law floor: 800 sqft for any ADU on any California single-family lot, regardless of lot-coverage or FAR rules, per Gov Code §65852.2(c)(2)(C).
- State-law ceiling for detached ADUs: 1,200 sqft if the city has adopted the expanded by-right size. Cities can adopt smaller maxima down to 850 sqft (1 bedroom) or 1,000 sqft (2+ bedroom), but not lower than that.
- JADU (Junior ADU): capped at 500 sqft, must be contained within the existing main house footprint, and requires owner occupancy of either the main house or the JADU.
- Lot coverage and FAR caps narrow the maximum from above on most lots. The 800 sqft state-law floor overrides these caps.
- Second-story rules add a separate constraint in many cities and often cap the second-story ADU at a smaller size than the ground floor.
The Floor Every City Must Respect
Gov Code §65852.2 sets size minimums that no California city can go below. These numbers are the foundation of every size conversation:
- Attached ADU (sharing a wall with the main house): state law guarantees a maximum of at least 850 sqft for a studio or 1-bedroom, and 1,000 sqft for a 2+ bedroom unit. The local maximum can go higher but not lower than these.
- Detached ADU: state law guarantees a maximum of at least 1,200 sqft, but local code can cap detached ADUs at the same 850/1,000 sqft thresholds if the city has not adopted the expanded by-right.
- 800 sqft “by-right” minimum: per Gov Code §65852.2(c)(2)(C), a city cannot use lot-coverage, FAR, or open-space rules to prevent an 800 sqft ADU on any single-family lot. This is the single most important number for tight lots.
- JADU (Junior ADU): capped at 500 sqft maximum by state law. Must be contained within the existing single-family residence (no new detached footprint). Requires owner occupancy of either the main house or the JADU.
- Conversion ADUs: when an ADU is created from an existing structure (garage, basement, attic), state law allows the ADU to match the existing structure’s full size, regardless of local caps — even if the structure exceeds 1,200 sqft.
What this means in practice: on the vast majority of California single-family lots, you can build at least 800 sqft without local interference. Many lots can build more. The exact ceiling depends on the next three layers.
Cities Can Be More Generous
State law is the floor, not the ceiling. Cities have wide discretion to allow ADUs above the state-law minimums. Los Angeles, San Diego, San Francisco, Oakland, and most major California metros have adopted local rules that allow detached ADUs up to 1,200 sqft as the published default.
Some cities go further:
- Los Angeles allows detached ADUs up to 1,200 sqft by right under LADBS ordinance. Two-story is permitted in most R1 zones with setback adjustments.
- San Diego through San Diego DSD allows detached ADUs up to 1,200 sqft and runs a pre-approved-plan program that fast-tracks plan check.
- Oakland allows up to 1,200 sqft detached and has expanded JADU rules.
- Some smaller jurisdictions publish higher local caps (1,500 sqft is not unheard of), but these are exceptions.
What this layer never does is reduce the state-law floor. If your city publishes a 750 sqft maximum, the state-law 800 sqft floor controls. The local cap loses.
The free Reality Check reads the local maximum for your address and returns the published number.
The Ceiling From Above
Even when state and local rules allow 1,200 sqft, the actual maximum on your lot is often smaller because of two geometric constraints:
- Lot coverage cap. Most California single-family zones cap total lot coverage (main house + garage + ADU + other hard structures) at 40% to 50% of lot area. On a 5,000 sqft lot with a 40% cap, total hard footprint cannot exceed 2,000 sqft. If your main house and garage already occupy 1,800 sqft of ground area, you have 200 sqft of remaining ground coverage — which forces either a two-story ADU or a footprint smaller than the local maximum.
- Floor-area ratio (FAR). Some cities, including Los Angeles in certain neighborhoods, layer an FAR cap on top of lot coverage. FAR is total building floor area (across all stories) divided by lot area. A 0.5 FAR cap on a 5,000 sqft lot means total floor area cannot exceed 2,500 sqft. A two-story ADU consumes FAR twice as fast as a single-story one.
The 800 sqft state-law preemption rescues most tight lots from these caps. Above 800 sqft, the geometry binds. A practical example: a 6,000 sqft lot with a 2,200 sqft main house, 400 sqft garage, and 40% lot-coverage cap. Total allowed coverage is 2,400 sqft. Remaining ground coverage is 2,400 minus 2,600 — already over. Under standard rules, no additional ground coverage is permitted. The 800 sqft state-law preemption forces the city to allow an 800 sqft ADU anyway. Above 800 sqft, the homeowner must either convert existing space (garage, basement) or build a two-story ADU that uses minimal ground footprint.
A Separate Ceiling
Two-story ADUs are allowed under California state law on most single-family lots, but cities add specific second-story rules that act as a fourth layer:
- Setbacks for second story. Some cities require larger second-story setbacks than ground-floor setbacks. A 4-foot rear setback on the ground floor may step back to 8 or 10 feet on the second floor, which shrinks the second-floor footprint.
- Height caps. State law allows 16 feet for a detached one-story ADU and up to 25 feet for a detached two-story ADU adjacent to single-family use. Local rules can be more permissive.
- Privacy setbacks. Some cities require additional setbacks where second-story windows face a neighbor’s yard.
- Second-story coverage caps within FAR. A two-story ADU consumes FAR at double the rate of a single-story footprint. Tight FAR caps often force a two-story design to be smaller than the local maximum.
The practical effect: a two-story 1,200 sqft ADU is often allowed but rarely fits cleanly without redesign. Most California two-story ADUs land at 900 to 1,100 sqft total, distributed roughly 60% ground floor and 40% second floor.
ADU Size Rules, California 2026
California state law guarantees an 800 sqft ADU on every single-family lot, regardless of local lot-coverage or FAR rules, per Gov Code §65852.2(c)(2)(C).
Detached ADU state-law maximum is 1,200 sqft under expanded by-right rules. Cities cannot cap below 850 sqft (1-bedroom) or 1,000 sqft (2+ bedroom).
JADUs are capped at 500 sqft by state law and must be contained within the existing main house footprint.
Conversion ADUs are allowed at the full existing-structure size, even above 1,200 sqft, when the ADU is created from an existing garage, basement, or attic.
California issued ~25,000 ADU permits in 2022, with the median permitted size landing in the 600 to 850 sqft band (CA YIMBY ADU Reform Retrospective, January 2024).
ADU Size Limits — California
The size your lot allows and the size that makes financial sense are often different numbers. The largest legal ADU is not always the best ADU for your goals.
- Rental income. The rent-to-cost sweet spot in most California metros is 600 to 800 sqft. Going from 800 to 1,200 sqft typically adds $80K to $140K in cost for $400 to $700 in additional monthly rent. The IRR rarely justifies the upsize.
- Aging-parent suite. 500 to 700 sqft handles single-occupancy comfortably for 10+ years. Building larger creates rooms that will not get used.
- Adult-child suite. 400 to 600 sqft is the typical sweet spot. Larger sizes erode the privacy buffer between generations.
- Primary residence + main-house rental. This is the one configuration where the maximum often is the right size, because the homeowner is moving in and the ADU is the primary unit.
We see this regularly: a homeowner gets cleared for a 1,200 sqft detached ADU, designs to 1,200 sqft because it is allowed, and overshoots the rental yield their neighborhood can support by $90K to $140K. The right answer was 800 sqft. Sometimes the right answer is not to build to the maximum — and we say that clearly, before the design fees are committed.