What Is an ADU? Types, Costs, and Permits Explained (2026)

What is an ADU?

An accessory dwelling unit (ADU) is a secondary, independent living space located on the same parcel as a primary residence. To count as a dwelling unit, it generally must include its own provisions for living, sleeping, cooking, and sanitation — in other words, a kitchen and a bathroom, not just an extra room.

ADUs go by many names depending on the region and the form they take: granny flats, in-law units, backyard cottages, secondary suites, and casitas. The unifying feature is that the ADU is accessory to a main home: smaller in scale, on the same lot, and usually under the same ownership. They are popular because they add housing capacity, can generate rental income, and let families house aging parents or adult children close by.

The main types of ADU

Detached ADUs (sometimes called DADUs) are standalone structures separate from the main house — the classic backyard cottage. They offer the most privacy and flexibility but typically cost the most because they are new construction from the ground up.

Attached ADUs share at least one wall with the primary residence, like an addition with its own entrance. Junior ADUs (JADUs) are a special, smaller category created within the existing footprint of the main house — often a converted bedroom — usually capped at a small size and sometimes allowed to share some facilities. Garage conversions turn an existing garage into living space, and basement conversions do the same below grade. Conversions of existing space are often the most affordable path because the shell already exists, though they can carry their own complications around egress, ceiling height, and utilities.

How much does an ADU cost, and how long does it take?

Cost and timeline vary enormously by type, location, size, finishes, and local fees, so treat any figure as a broad range rather than a quote. Conversions of existing space tend to be the least expensive, while detached new-construction units typically cost the most. Site conditions, utility connections, and impact or development fees can move the total significantly in either direction.

Timelines also vary widely. Design and permitting can take anywhere from a couple of months to much longer depending on how the jurisdiction reviews ADUs, and construction adds several more months on top of that. Streamlined or ministerial review processes — where they exist — can shorten the approval stage considerably. The honest answer is that costs and timelines are highly jurisdiction-dependent; always price against your specific city, lot, and design. (This is general information, not financial or legal advice.)

Do you need a permit for an ADU?

Yes. Building a habitable ADU is construction, and construction of a dwelling unit requires a building permit essentially everywhere. The process generally involves preparing plans, submitting an application to the local building or planning department, paying fees, going through plan review, and passing inspections during and after construction.

Beyond the building permit itself, ADUs often touch zoning, setback, parking, and utility-connection rules, which is where local variation really shows up. Some jurisdictions have adopted streamlined or by-right approval for qualifying ADUs to encourage them; others apply more discretionary review. The takeaway: budget time and money for permitting, and confirm the exact requirements with your local authority before you design. For more on the permit process itself, see our contractor's guide to building permits.

How ADU laws vary by state

ADU rules are set primarily at the state and local level, and they differ dramatically. Some states — California most prominently — have passed laws specifically to make ADUs easier to build, limiting how much cities can restrict them and creating streamlined approval paths. Other states leave far more discretion to individual municipalities, so what's allowed in one city may be prohibited in the next town over.

Because of this, the single most important step is to check the rules where you actually plan to build. You can explore permit activity and coverage state by state on our coverage hub, including a detailed look at California ADU and permit data, where ADU-friendly legislation has driven some of the highest activity in the country. (Informational only — not legal advice; verify current rules with your jurisdiction.)

Frequently asked questions