ADU setback requirements are set by your city in most states, but a handful of state laws cap how strict a city can be. California limits side and rear setbacks to 4 feet for a new ADU, Arizona caps them at 5 feet, and Colorado ties them to the main house and a 5-foot rear limit. Washington, Massachusetts and Montana say an ADU can’t face stricter setbacks than the main house. Texas, Florida and most other states leave the number to local code.
Two other rules matter as much as the zoning number. Converting an existing building often keeps its current position, and the building code’s fire-separation rules apply to any wall within 5 feet of a lot line. This guide reflects sources checked in October 2026.
ADU setback requirements by state at a glance
| State | What state law says about ADU setbacks | Who sets the number |
|---|---|---|
| California | Side and rear: 4 ft maximum for attached and detached ADUs. None for conversions or a rebuild in the same spot and size | State cap; city sets front |
| Arizona | Cities over 75,000 can’t require side or rear setbacks above 5 ft | State cap |
| Colorado | Side: no more than the main house’s. Rear: no more than the greater of 5 ft or the rear setback for other accessory buildings. Applies in covered metro areas | State ceiling |
| Washington | No stricter than for the principal unit; detached ADUs may sit at an alley lot line | Tied to main house |
| Massachusetts | Setbacks must be equal to or more permissive than for single-family homes | Tied to main house |
| Montana | No more restrictive than for single-family dwellings | Tied to main house |
| Oregon | Cities must allow an ADU subject to “reasonable” siting and design rules, with no fixed number | City |
| Maine, Vermont | Setbacks set locally; Maine attached ADUs match the house | City or town |
| Texas | No statewide ADU law, so no state setback rule | City, plus deed restrictions |
| Florida | Setbacks set by county or city code. Statewide ADU legislation was debated in 2026, so check its status | County or city |
| New York | No statewide ADU rule found; local codes control | Town or city |
Sources: California HCD; Arizona’s HB 2720; Colorado DOLA; Washington’s MRSC; Oregon ORS 197.312. Massachusetts comes from a Realtor summary, and Montana, Maine and Vermont from a 2024 Virginia Housing Commission review that predates some changes. Verify those with your city.
A state “cap” is a ceiling, not a guarantee of a specific number. Your city must still apply its other rules, and an ordinance that conflicts with state law may be void.
How setbacks are measured
A setback is the minimum distance between a structure and a lot line. Most codes set separate front, side, rear and street-side distances. Front setbacks usually stay local even where the state caps side and rear. California is one exception: a front setback can’t prevent an 800 sq ft ADU, even if the unit ends up in the front setback.
Three measurement points trip up designs:
- The true lot line. A fence can be off by inches or feet. A boundary survey settles it before you design.
- Wall or eave. Some codes measure to the wall, others to the roof edge. Confirm which one your city uses.
- Easements. A sewer, power or drainage easement can block building even where the setback allows it.
A worked example
The numbers below are hypothetical, chosen only to show the arithmetic. Say a lot is 50 ft wide and the house is 30 ft wide, leaving 20 ft of side yard to split. A detached ADU behind the house is limited by the side setbacks on each side of the lot, not by the house. With 4-ft side setbacks the ADU can be up to 42 ft wide. With 10-ft side setbacks it can be up to 30 ft wide. Eaves, a rated wall and easements would then shrink that further, which is why the number from your own city and a survey matter more than any state average.
New ADU or conversion: the footprint exception
California is the clearest example. A city can’t require a setback for an ADU or JADU created within an existing living area or accessory structure, or for a new structure built in the same location and to the same dimensions as an existing one. HCD says conversions still follow building, health and safety, and fire standards, and one converted from an accessory structure can add up to 150 sq ft for stairs or an entry.
Two corrections to common advice. HCD’s text does not require the old structure to stay standing, and a same-footprint rebuild qualifies. It also does not say the existing building must be permitted, so ask your city how it treats an unpermitted garage. Our garage conversion cost guide covers budgeting.
Washington’s HB 1337 also blocks cities from preventing a conversion of an existing structure that violates current setbacks, according to the Virginia review. Elsewhere, assume a conversion must meet current setbacks unless the local ordinance says otherwise.
Fire separation: the building-code rule behind the setback
Zoning setbacks and fire separation are different tests. The International Residential Code, used in most states with local amendments, regulates walls close to a lot line. For dwellings without sprinklers, typical tables require:
- walls less than 5 ft from the lot line to carry a 1-hour fire-resistance rating
- no openings (windows or doors) in a wall less than 3 ft from the lot line
- openings limited to 25 percent of the wall area at 3 ft to under 5 ft
- roof overhangs between 2 ft and 5 ft to have a 1-hour rated underside
Fire separation distance can also be measured to an imaginary line between two buildings on the same lot. California and other states amend the model code, and sprinklered homes follow a second table, so ask your building department for the version it enforces. Our fire sprinkler guide covers California specifics.
A 4-foot setback can therefore be legal on the zoning side and still need a rated wall, no windows facing the lot line, and restricted eaves. Ask your designer to price that early, then check the numbers with the ADU Cost Calculator.
How to find the setback for your lot
- Identify your state’s rule in the table, then find your city’s ADU ordinance or zoning code. The Zoning Information Lookup is a starting point.
- Check whether the unit is a conversion, attached, or new detached. Setbacks often differ.
- Order a boundary survey and pull easements from your title report.
- Ask the city whether it measures to the wall or eave, and what the fire rules require. A short pre-application meeting with planning and building staff often settles both questions before you pay for drawings.
- Confirm private limits. In Texas, deed restrictions can impose setbacks that city code doesn’t. See our Texas ADU permit guide and HOA rules.
Texas examples show how local it gets. San Antonio’s own page says an ADU can’t have an eave overhang within 3 feet of the property line and needs fire-rated construction within 5 feet. A secondary summary lists 5-foot side and rear setbacks for detached units there, and a 3-foot side setback in Dallas ADU-overlay areas. Those figures need city confirmation.
Mistakes that send plans back
- Measuring from a fence and not the surveyed line.
- Assuming a state cap applies to the front yard.
- Counting the wall setback but forgetting the eave.
- Skipping the fire-rated wall in the budget.
- Assuming a conversion is exempt without confirming the local rule.
- Planning a window on a wall facing the lot line.
For a deeper review of why plans bounce, see why ADU permits get rejected and our site plan requirements.
Frequently asked questions
Can I build an ADU right on the property line?
Usually not for a new unit. California allows zero setback only for conversions and same-footprint rebuilds, and fire rules limit what a wall within 5 feet can contain. Washington allows detached ADUs at an alley lot line.
Do roof eaves count toward the setback?
It depends on how your city measures. Separately, the building code limits projections near the lot line, including 1-hour protection between 2 and 5 feet in the model tables.
Do state laws change side and rear setbacks for attached ADUs?
In California, the 4-foot limit covers attached and detached ADUs. In Washington, Massachusetts and Montana, an attached ADU follows the main house’s setbacks. Check your city for other states.
Next steps
Confirm your state’s rule, measure from a survey, and ask the city how it treats eaves and fire separation. The AI ADU Design Advisor can show layouts that fit your buildable area, and the Contractor License Lookup helps you check a builder’s license. See also our California ADU laws and Florida ADU laws guides.
