What is an ADU?

The definition

An accessory dwelling unit (ADU)is a self-contained living space on the same property as a single-family home (and increasingly, multifamily buildings). “Self-contained” is the key word: a true ADU has its own kitchen, bathroom, sleeping area, and entrance. A bedroom with a mini-fridge is not an ADU; a 500 sq ft backyard cottage with a full kitchen is.

Because an ADU is legally accessoryto the main house, it generally can't be sold separately (though some states are starting to allow condo-style separate sales). It shares the lot, and often utilities, with the primary residence.

One idea, many names

Cities and regions use different words for the same thing. All of these are ADUs:

• Granny flat / in-law unit: the classic use case, housing a family member close by but independently.
• Casita: common in the Southwest; Spanish for “little house.”
• Backyard cottage / laneway house / carriage house: regional names for detached units.
• Secondary suite / basement apartment: units carved out inside the primary home.
• DADU: “detached ADU,” used in codes like Seattle's to distinguish backyard units from internal ones.
• JADU (junior ADU): a California category, a unit of 500 sq ft or less created withinthe walls of the existing home, allowed an “efficiency kitchen” and sometimes a shared bathroom.

When you search your city's municipal code, try several of these terms; older ordinances often say “secondary dwelling” or “accessory apartment.”

Why cities regulate ADUs

Zoning codes control what can be built where, and ADUs sit at the intersection of several things cities care about: neighborhood character, parking, utility capacity, and rental housing supply. Typical rules cover:

• Size limits: maximum square footage, often 800–1,200 sq ft for detached units, sometimes tied to a percentage of the main house.
• Setbacks: how far the unit must sit from property lines (4 ft side/rear is a common modern standard).
• Height: one story (16 ft) is a common baseline, with two-story allowances appearing in newer codes.
• Parking: some cities require a replacement space; many states now prohibit parking requirements near transit.
• Owner-occupancy: whether the owner must live in the main house or the ADU. Many states have suspended or repealed these requirements.
• Short-term rental restrictions: whether you can put the unit on Airbnb (frequently: no, or only with a permit).

This is exactly why the rules are so hard to pin down: your project is governed by state law, county code, and city zoningsimultaneously, and they change on different schedules. That's the problem our directory exists to solve: every jurisdiction we cover links to its official, primary sources.

Why ADUs are booming

Three forces converged over the last decade. First, the housing shortage pushed states (California most aggressively) to override restrictive local zoning and legalize ADUs statewide. Second, multigenerational living rebounded: an ADU lets aging parents or adult children live close but independent. Third, the economics work: a unit that costs $150k–$300k to build can rent for $1,500–$3,000/month in strong markets while adding meaningful resale value.

In California alone, ADU permits grew from roughly 1,000 per year in 2016 to over 20,000 per year, about one in five new homes in the state. Oregon, Washington, Montana, Vermont, and a growing list of states have followed with their own ADU-enabling laws.

Where to start

1. Look up your city's official rules to confirm an ADU is allowed on your lot and note size, setback, and parking requirements.
2. Read up on the types of ADUs to figure out which fits your property and budget.
3. Sanity-check the money: what ADUs cost and how to finance one.
4. Call your planning department. Ten minutes with a planner (many cities offer free counter appointments) will save you weeks.