A setback is the distance a building must stay from a property line. Lot coverage is the share of your lot that buildings may cover. Floor area ratio (FAR) compares total building floor area to lot size. Open space rules require part of the lot to stay unbuilt. Normally these rules shrink what you can build. For ADUs, state law limits how far a city can go.
LA ADU Guide is an information site. We are not a contractor, architect, lender or law firm. Rules change, so confirm everything with your building department before you spend money. This page is general information, not legal advice.
The four-foot rule
Under Gov. Code section 66323(a), a city "shall ministerially approve" a new detached ADU on a single-family lot with side and rear setbacks of no more than four feet, within the size limit for that category. ABAG's 2025 summary says state exempt units "may not be subjected to" local standards such as lot coverage, front setbacks or design standards.
A city may require less than four feet. It may not require more for a state exempt ADU.
Setback rules by situation
| Situation | State rule |
|---|---|
| New detached ADU, state exempt | Side and rear setbacks no more than 4 feet |
| Conversion of an existing room, garage or accessory structure | No additional setback; the existing footprint can be used if sufficient for fire and safety |
| Rebuild of an existing structure in the same location and dimensions | No additional setback |
| Other ADUs, front setbacks | May be applied, but cannot "preclude an ADU of at least 800 square feet" |
| JADU | Inside the home, so no new setback |
Source: ABAG summary, tables 2 and 4. Section 66321 covers the non-exempt standards.
Lot coverage, FAR and open space
The ABAG summary states that a jurisdiction "may not impose FAR, lot coverage, size, front setback, or open space requirements that do not permit at least an 800 square foot ADU with 4-foot side and rear setbacks."
In plain terms:
- If your lot is already near its coverage limit, the city may still need to let an 800 square foot ADU go forward.
- Cities may apply coverage and open space limits to larger ADUs, if the standards are objective and written in the local ordinance.
- A coverage cap cannot be used to deny a state exempt ADU at all.
HCD's 2024 letter to Los Angeles repeated this point. It said front setbacks "cannot be imposed" on ADUs and JADUs created under section 66323(a), and that a city can only prohibit an ADU in a front setback if the ADU is larger than 800 square feet or does not meet the side and rear rule. It told the City to fix its ordinance so zoning review and front setbacks do not prevent an 800 square foot ADU.
Fire and safety still apply
A setback of a few feet is a zoning rule. Building and fire codes are separate. Close to a property line, you may need fire-rated walls, limited windows or eaves, and in mapped fire zones other measures. Our fire hazard zones guide covers the main points. The building department decides what applies to your plans.
Easements, utilities and trees
Setback rules do not erase easements. A utility, drainage or access easement on your title can still block construction in that strip. Check your title report or ask the county recorder. Also look for sewer lines, gas meters, trees with protected status and slopes. These affect where a unit can really go even if the setback math works.
Common mistakes
- Measuring from the fence instead of the legal property line. Fences are often off.
- Assuming a corner lot has the same setbacks as an interior lot. Street side setbacks can differ.
- Forgetting eaves, stairs or decks, which a city may count as part of the structure.
- Treating the 800 square foot floor as a target for every unit. It is a minimum a city must allow, not a limit.
Checklist
- [ ] Get a survey or plot plan with the legal property lines.
- [ ] Mark the four-foot side and rear strips on the plan.
- [ ] Check for easements in your title report.
- [ ] Ask the building department whether your ADU qualifies as state exempt.
- [ ] If it does not, ask for the local setback, coverage, FAR and open space standards in writing.
- [ ] Ask whether any overlay (hillside, fire, coastal, historic) adds rules.
Common questions
Can my city require a bigger setback than four feet? For a state exempt detached ADU, no. Section 66323 caps side and rear setbacks at four feet. A city may require less.
Does a conversion need a setback? ABAG says conversions and rebuilds need "no additional setbacks," as long as the existing footprint is sufficient for fire and safety.
What if my lot is small? The state floor protects an 800 square foot ADU with four-foot side and rear setbacks. If your lot cannot fit even that, a local standard may still apply. Ask the city to show the math in writing.
Who decides? The permitting agency applies the rules, and HCD reviews city ordinances for conflicts with state law. We cannot tell you how a plan checker will read your lot.
Recent change
Front setbacks joined the protected list through AB 2221 (2022), according to HCD's review letter. Section numbers changed in 2024 when the law moved to sections 66310 to 66342 (SB 477, effective March 25, 2024). For sizes and heights see ADU size and height limits, and for the overall law see California ADU rules explained.