The short answer: There is no national ADU law in the United States. Whether you can build an ADU (accessory dwelling unit) on your property depends on your state, your city’s zoning and your specific lot. In many places the answer is now “yes, by right,” but five checks decide it for your property. Do them in this order, because each one can overrule the next.
Last verified: October 5, 2026 · Written by the ADU Scope editorial team
Step 1: Can you build an ADU on your property under state law?
A state law sets the floor: your city can be more permissive, but it cannot go below it. Four examples show how different the floors are:
| State | What the state requires | Source |
|---|---|---|
| California | Cities must approve a converted ADU, a JADU and an 800 sq ft detached ADU on a typical single-family lot; no owner-occupancy requirement for ADUs | HCD ADU Handbook, March 2026 |
| Oregon | Cities over 2,500 people and counties over 15,000 must allow at least one ADU per detached single-family home inside urban growth boundaries; no owner-occupancy or added-parking requirements | ORS 197A.425 |
| Washington | Cities and counties planning under the Growth Management Act must allow two ADUs per lot in urban growth areas; no owner-occupancy requirement; impact fees capped at 50% of the main unit’s | MRSC |
| Massachusetts | Since February 2, 2025, cities and towns must allow the first ADU by right in single-family zones; owner-occupancy requirements are prohibited and parking is limited | MAPC, MGL c. 40A § 3 |
If your state isn’t listed here, it may still have a statewide law, or it may leave everything to cities. Our state-by-state guide covers the rest.
Step 2: Can you build an ADU on your property under city zoning?
Find your lot’s zoning district on the city’s online zoning map, then read the city’s ADU ordinance. Look for four things:
- whether ADUs are allowed in your district
- the maximum size
- whether attached, detached or both are permitted
- whether the city requires a discretionary hearing or approves them over the counter
If the city never adopted an ADU ordinance, state law (if any) governs on its own.
Step 3: Can you physically build an ADU on your property?
Legal permission doesn’t guarantee your lot fits an ADU. Check:
- Setbacks and height: California’s statewide floor is 4 ft at the side and rear and 16 ft tall for a detached ADU, but other states and cities differ.
- Space: the rear yard must fit the unit plus access and fire clearances.
- Easements and utilities: sewer, water and power lines can block a location. If you’re on a septic system, capacity is often the deciding factor.
- Existing structures: a garage, shed or basement may be convertible for much less than a new build.
Step 4: Does an HOA or deed restriction block it?
Private covenants (CC&Rs) can ban ADUs even where zoning allows them. Some states limit this: California, for example, voids covenants that effectively prohibit or unreasonably restrict ADUs. Others leave it to the covenant. Read your CC&Rs before you pay for plans.
Step 5: Do any special overlays apply?
Check for:
- coastal zones
- historic districts
- floodplains
- high fire-hazard areas
- land outside an urban growth boundary (the Oregon and Washington rules above apply only inside them)
These are where a “yes” at the state level most often turns into extra review or a “no.”
Can you build an ADU on your property? The California types of ADU
Under California Government Code section 66323, a city must approve four categories of ADU by a ministerial process, as the state housing agency (HCD) summarizes in its March 2026 ADU Handbook:
- A conversion on a single-family lot: an ADU created inside the existing home or an accessory structure, with up to 150 sq ft added for entry and exit. Setbacks can only be those needed for fire and safety.
- A new detached ADU on a single-family lot: up to 800 sq ft of interior living space, with 4 ft side and rear setbacks. Height starts at 16 ft, with higher allowances near transit and for some roof designs, so check the handbook table for your lot.
- Conversions on a multifamily lot: non-livable space such as garages, storage rooms or basements, up to 25 percent of the existing units and at least one.
- Detached ADUs on a multifamily lot: up to eight, but no more than the number of existing units; two on a lot where multifamily housing is only proposed.
A Junior ADU (JADU) is separate: up to 500 sq ft inside a single-family home, with an efficiency kitchen. If your plan fits one of these categories, your city generally has to approve it even if its own ordinance is stricter.
What a city cannot require when you ask to build an ADU on your property
The practical value of the state floor is the list of things a city cannot demand. For a California project, these are the ones that most often change the answer:
- Discretion. Review is ministerial and uses objective standards. A city decides within 60 days of a complete application or the ADU is deemed approved (Gov. Code section 66317).
- Owner occupancy for an ADU. It cannot be required. The exception is a JADU that shares a bathroom with the main home.
- Parking. None can be required within half a mile of transit, in a historic district, when the ADU is part of the main home or an accessory structure, when the city requires on-street permits and does not issue them, or within one block of a car-share location. Where parking can be required, the cap is one space per ADU or bedroom, whichever is less.
- Fire sprinklers. Not required for the ADU unless the main house is required to have them.
- Impact fees on small units. None on an ADU of 750 sq ft or less. Larger units pay in proportion to the size of the main home.
- Design and lot standards for the four categories above. The state bars design standards, lot coverage, floor-area ratio and open-space rules for these categories.
A homeowners association is different. California Civil Code section 4751 voids covenants that effectively prohibit or unreasonably restrict ADUs and JADUs. Our guide on whether your HOA can block an ADU covers the details.
Reasons you may not be able to build an ADU on your property
A state “yes” can still hit a physical or regulatory wall. These are the real-world reasons a lot fails, and they are worth checking before you pay a designer:
- No adequate water or sewer service. Colorado’s statewide ADU law, for example, carves out lots with no water or sewer service or an inadequate well. Septic lots elsewhere often depend on system capacity.
- Flood hazard. The Colorado law also excludes lots in a floodway or FEMA 100-year floodplain, and other jurisdictions apply their own floodplain rules.
- Lot geometry and fire access. A rear yard has to fit the unit, the walking path to it and the clearances fire officials expect.
- Easements and utility lines. You generally cannot build over a recorded easement, a sewer lateral or an overhead power line.
- Overlays. Coastal zones can require a coastal development permit, and historic districts add design review in many cities.
- Outside the urban boundary. The Oregon and Washington mandates apply only inside urban growth boundaries or areas.
What to ask the planning department
Before you hire anyone, call or visit and ask three questions:
- Which ADU types can I build on this parcel (conversion, attached, detached)?
- What is the maximum size and the required setbacks?
- Are there any overlays, easements or restrictions on this parcel?
Get the answers in writing, or ask for a pre-application meeting. Bring your parcel number, a survey or plot plan and a sketch of where you want the unit, and ask the planner to confirm the answers by email so you can point to the code section behind any “no”.
FAQ
Can I build an ADU if I rent my home?
Not without the owner’s written permission. The owner is the one who applies.
Do I need to live on the property?
In California, Oregon, Washington and Massachusetts, the state law bars owner-occupancy requirements for ADUs. Elsewhere, check your city.
How do I find my zoning district?
Search “[your city] zoning map” and enter your address. If there is no online map, the planning department can tell you by phone.
What if my city says no but my state says yes?
Ask the city to cite the code section. If it conflicts with state law, appeal to the city first, then escalate to your state housing agency.
Next steps
- ADU Laws by State: Where You Can Build in 2026
- California ADU Laws 2026: What Changed and What You Can Build
- How Long Does It Take to Get an ADU Permit in California?
- Can My HOA Block an ADU? Rules by State
- ADU Setback Requirements Explained: California, Washington, Colorado and Portland
- ADU Impact Fees: When You Pay Them and How to Avoid Them
General information, not legal advice. Local rules vary and change; confirm with your planning department.
Sources: HCD ADU Handbook (March 2026) · ORS 197A.425 · MRSC, Accessory Dwelling Units (Washington) · MAPC, Accessory Dwelling Units in the Commonwealth · Cal. Gov. Code 66314 · Colorado DOLA, ADUs