Module 5 · Lesson 3
The 800 SF Rule: Your Guaranteed ADU
No matter what local lot-coverage or floor-area rules say, California guarantees you can build an 800 SF ADU. Here's how the rule works.
If you remember one number from this course, make it 800.
The rule: every city must allow an ADU of at least 800 square feet (up to 16 feet tall, with 4-foot side/rear setbacks) — and local development standards cannot be used to prevent it.
That second half is the teeth. Cities normally limit how much of a lot you can cover (lot coverage), how much floor area a lot can hold (FAR), and how much open space must remain. For any other structure, maxed-out numbers mean "no." For an 800 SF ADU, the law says those standards must yield.
What this means in practice:
- Your lot is "already at maximum coverage"? You can still build 800 SF.
- Your city requires 40% open space and you'd drop below it? You can still build 800 SF.
- The front setback leaves nowhere else to put it? The front setback bends — you can still build 800 SF.
What the rule doesn't do: it doesn't override physical reality (easements, the fire code, your actual lot dimensions), and it doesn't guarantee anything above 800 SF — bigger units follow the normal standards from lesson 4.1. Think of 800 SF as the unit you can build even when everything else says no.
Key takeaways
- Every lot gets at least an 800 SF ADU — lot coverage, FAR, and open-space rules must yield.
- The guarantee includes 16 feet of height and 4-foot setbacks.
- It doesn't override easements, safety codes, or physics — just zoning math.
- Above 800 SF, normal local standards apply again.