ADU Law by State

State accessory dwelling legislation is written in a fairly consistent pattern, and knowing the pattern lets you evaluate your own situation quickly.

The statute first defines which jurisdictions it binds. Some apply statewide; others apply only to municipalities above a population threshold, or only to those planning under a growth management framework. Montana's applies to cities over 5,000; Arizona's to municipalities over 75,000. If your property sits outside the covered class, the statute does not help you and local zoning governs.

It then states what must be permitted — typically one unit per single-family lot, sometimes two, sometimes with a size ceiling such as 800 or 900 square feet. Units at or under the ceiling get the statutory protection; larger units revert to local discretion.

It then removes specific local barriers. The four that matter are owner-occupancy requirements, off-street parking mandates, minimum lot size conditions, and impact fees. A statute that requires the unit but permits a parking mandate has left the jurisdiction a tool that can still make a project physically impossible on a narrow lot.

Finally it sets a review clock and a standard of review. Ministerial or by-right review means objective standards and no hearing. Discretionary review means a hearing, which means neighbors, which means schedule risk regardless of what the statute nominally requires.

What no statute does is compel utility capacity, override recorded deed restrictions, or bind a homeowners association. Those three are the most common reasons a project that is legal under state law still cannot be built on a particular parcel.

Thirty-five states have no state-level accessory dwelling mandate, which does not mean accessory units are prohibited there. It means the decision sits entirely with the municipality, and the range of outcomes is wide.

In practice three patterns show up. Some cities in non-statute states have written permissive accessory unit ordinances on their own initiative, often as part of a housing element or a downtown density plan — these can be as workable as anything in a statute state. Others permit accessory units only as a conditional use, which means a hearing, a notice period and neighbors with standing to object; the unit may be approvable but the schedule is unpredictable. And some simply have no accessory dwelling category at all, in which case the question becomes whether a second unit can be permitted under some other classification such as a guest house without a kitchen, or whether the parcel can be rezoned.

The research path in a non-statute state is short: pull the zoning ordinance for the specific parcel, search it for "accessory dwelling," "accessory apartment," "guest house" and "secondary dwelling," and call the planning counter before you spend anything on design. Fifteen minutes on the phone with a planner who knows the parcel is worth more than any amount of general research.

Two constraints apply everywhere regardless of statute. Recorded deed restrictions and homeowners association covenants are private agreements that state law generally does not override, and they are the most common reason a project that is fully legal under zoning still cannot proceed. And no statute compels a utility district to provide capacity that does not exist.

Accessory dwelling legislation has moved in one direction for roughly a decade, and the pattern of adoption is fairly predictable. States facing acute housing cost pressure and constrained buildable land act first, usually beginning with a permissive statute that preserves substantial local discretion, then returning in a later session to close the loopholes that discretion created.

California is the clearest illustration of that arc. The first accessory dwelling statutes left jurisdictions enough latitude that permitting remained difficult in practice, and the legislature came back repeatedly — tightening the approval clock, removing owner-occupancy conditions, prohibiting impact fees under a size threshold, and eventually addressing lot splits directly. Each round removed a specific tool that jurisdictions had used to slow projects down.

The practical implication for anyone planning a project is that the rules in a statute state are more likely to loosen than tighten, and that a provision blocking your project today may not survive the next session. It also means the fifteen-state list is a snapshot rather than a fixed set. Several additional states have introduced accessory dwelling legislation without passage, and those bills tend to return.

Verify the current text before you rely on any summary, including this one. Statutes get amended, effective dates lag passage, and municipalities sometimes take a full cycle to update their own ordinances to match state law — which means the counter may quote you the old rule even after the new one took effect.