How Big Can My ADU Be?

California's size rules look like alphabet soup at first. They're actually just three layers: what's guaranteed, what's typical, and what your specific city allows.

Layer 1: The guarantees (cities can't go below these)

• 800 SF, always. Every city must allow an ADU of at least 800 square feet (up to 16 feet tall, with 4-foot setbacks) on virtually any single-family lot — regardless of local lot-coverage, floor-area, or open-space rules. If a local standard would prevent an 800 SF unit, the standard yields.
• 850 SF for a studio or one-bedroom. Cities can't cap qualifying ADUs below this.
• 1,000 SF for two or more bedrooms. Adding a bedroom raises the guaranteed floor.

Layer 2: The typical maximums

• Detached ADUs: most cities allow up to 1,200 SF; some allow more.
• Attached ADUs: up to 50% of the main house's living area — but never less than 800 SF. (Own a 1,400 SF house? Your attached ADU can still be 800 SF, not 700.)
• JADUs: capped at 500 SF within the existing home.

Layer 3: Your city

Within state limits, cities set their own maximums, and they vary a lot. Some allow 1,200 SF flat; some scale by lot size; a few generous cities go higher. This is a five-minute lookup in your city's ADU ordinance — or part of any feasibility review.

The number that matters most

For most homeowners, the practical envelope is 800–1,200 SF — and here's the reassuring part: even the most restrictive city in California must let you build 800 SF. That's a full two-bedroom's worth of space. Nobody is stuck with a closet.