ADU Eligibility
Can I Build an ADU on My Property?
How to find out whether your lot qualifies — the factors that decide it, the state laws that override local bans, and how to check your exact address.
The short answer
In most of the United States, you can build an ADU if you own a single-family lot — state laws in California, Washington, Oregon, Utah, Colorado, and Montana now override local bans. The factors that actually decide it are your zoning, lot size and setbacks, size limits, parking, utilities, and owner-occupancy rules.
What decides it
The 6 factors that decide if you can build
"Can I build an ADU?" always comes down to the same six questions. Answer them for your address and you know whether — and what — you can build.
Zoning
The first question is whether your lot's zoning allows an ADU at all. Single-family and multifamily residential zones almost always qualify. Six states — California, Washington, Oregon, Utah, Colorado, and Montana — now legalize ADUs by state law and override local bans in the areas they cover. Everywhere else, your city or county zoning code is the deciding document.
Lot size & setbacks
Setbacks — the required distance from your property lines — determine where an ADU can physically sit. California bars cities from imposing a minimum lot size for an ADU and caps side and rear setbacks at four feet. In states without a preemption law, minimum lot size and setback requirements are set locally and vary widely from one city to the next.
Size limits
Most jurisdictions cap ADU square footage. California guarantees at least 800 square feet regardless of local rules, while Montana caps a detached ADU at 75% of the main home or 1,000 square feet. Detached units, attached additions, and internal conversions (a garage or basement) each carry different size limits.
Parking
Off-street parking requirements quietly kill projects on tight lots. California and Oregon prohibit cities from requiring added parking for many ADUs, and California waives it entirely within half a mile of transit. States without an ADU law leave parking minimums to local code, where one or two required spaces can make a project impossible.
Utilities
An ADU needs water, sewer or septic, and electrical capacity. Homes on septic may need a new or upgraded system, and an older electrical panel often needs an upgrade to add a unit. California limits utility connection fees for ADUs under 750 square feet, but septic and panel work remain the most common hidden costs.
Owner-occupancy
Some jurisdictions require you to live on the property. California cannot require owner-occupancy for standard ADUs permitted between 2020 and 2025 (junior ADUs are an exception), and Oregon and Washington bar the requirement too. Utah, by contrast, requires the owner to occupy the primary home to add an internal ADU.
State-by-state
Where state law overrides local ADU bans
Six states have passed laws that force local governments to allow ADUs, which is why eligibility no longer depends only on your city. Everywhere else, your local zoning code is the deciding authority.
| State | Statewide law? | Key statute | What it means |
|---|---|---|---|
| California | Yes | Gov. Code §66310–66342 (SB 477, 2024; formerly §65852.2) | ADUs allowed by right on single-family and multifamily lots statewide, no minimum lot size, owner-occupancy waived for standard ADUs. HCD enforces. |
| Washington | Yes (in urban growth areas) | HB 1337 (2023) → RCW 36.70A.680–.681 | Up to two ADUs on single-family lots inside urban growth areas of GMA-planning cities. No owner-occupancy; garage conversions allowed. |
| Oregon | Yes (cities over 2,500) | SB 1051 (2017) + HB 2001 (2019) | At least one ADU per detached single-family home inside the urban growth boundary. No parking or owner-occupancy conditions. |
| Utah | Yes (internal ADUs) | HB 82 (2021) → Utah Code §10-9a-530 | At least one internal ADU permitted by right in residential zones. Owner must occupy the primary home; cities may exempt up to 25% of residential land. |
| Colorado | Yes (subject jurisdictions) | HB24-1152 (2024) | Front Range subject jurisdictions must allow one ADU by administrative approval (deadline June 30, 2025). HOAs cannot be more restrictive. |
| Montana | Yes | SB 528 (2023) | At least one ADU by right on single-family lots in zoned municipalities (effective Jan 1, 2024). Detached units capped at 75% of the main home or 1,000 sq ft. |
| Everywhere else | No statewide law | Local zoning code | Legality depends entirely on your city or county zoning. Check your municipal code or ask your local planning department. |
Sources: California HCD ADU Handbook and Gov. Code §66310–66342; Washington RCW 36.70A.680–.681; Oregon SB 1051 / HB 2001; Utah Code §10-9a-530; Colorado HB24-1152; Montana SB 528. State laws set the floor — cities can still add reasonable local standards, so always confirm the current rules for your address.
Your address
How to check your specific property
The rules above tell you the general picture. To know what your lot actually allows, you have three honest options.
Free, but slow
Call your local planning department
Planners can confirm your zoning and whether ADUs are allowed. Expect to wait on hold, get general answers, and still have to interpret the code and overlays yourself.
Accurate, but expensive
Hire an ADU designer or architect
A designer will give you a precise answer, but you are paying $2,000–$10,000+ for design work before you know whether the project even pencils out.
Instant and cited
Get an ADU feasibility report
ADU Pilot pulls your parcel's zoning, setbacks, size limits, hazard overlays, and the governing state and local rules into one source-cited report in minutes — so you know if and what you can build before spending on design.
Get the answer for your address in minutes.
ADU Pilot turns your address into a source-cited feasibility report — zoning, setbacks, size limits, hazard overlays, and the governing state and local rules — so you know whether you can build before you spend on design.
FAQ
Common ADU eligibility questions
Do I need a permit to build an ADU?
Yes. Every ADU requires a building permit. In states with an ADU law, such as California, approval is ministerial — there is no public hearing or neighbor vote, and the city must approve or deny a complete application within 60 days. In areas without a state law, an ADU may still require discretionary review.
How long does ADU approval take?
Permit review commonly runs one to three months. California caps ministerial ADU review at 60 days once your application is complete. Construction then adds several more months. Incomplete applications and utility or septic issues are the most frequent sources of delay.
Can my HOA block an ADU?
In California, generally no. Civil Code §4751 voids any HOA covenant that prohibits or unreasonably restricts an ADU on a single-family lot, though reasonable design and aesthetic restrictions still apply. Colorado's ADU law similarly limits HOAs. In states without such a law, an HOA may still be able to block one.
Can I build an ADU if my lot isn't single-family?
Often yes. California and Washington allow ADUs on multifamily and mixed-use lots as well as single-family lots. The single-family lot is simply the most common and clearest case — many other zoning designations qualify too.
Will building an ADU raise my property taxes?
Yes, but only the added value is reassessed. Under California's Proposition 13, the ADU is assessed as new construction while your existing home keeps its original base-year value. Your whole property is not reassessed.
What's the difference between an ADU and a JADU?
A junior ADU (JADU) is carved out of the existing home's footprint, usually capped around 500 square feet, and typically requires the owner to live on-site. A standard ADU can be detached or attached, is larger, and has looser occupancy rules.
