ADU permits, explained

Does an ADU need a permit?

Always. An accessory dwelling unit is a second home on your lot, and cities regulate it as one. Seattle says it plainly: ADUs are not legal unless they have been established through a permit process, whether the unit is inside the house or detached in the yard^[1]. Austin, Texas, defines a dwelling unit as any structure or part of one with a habitable space, a full bathroom and permanent cooking facilities, and applies to it the rules for dwelling units, including zoning, utility meter, internal access, occupancy and address requirements^[2].

That definition is why a finished basement or a garage studio can become an ADU without anyone planning it. Monticello, Minnesota, says creating a full kitchen in the basement of a single-family home must be reviewed for zoning^[5].

Attached, detached or converted

The kind of ADU changes the permit. Seattle calls a legally permitted unit inside the home an attached accessory dwelling unit (AADU) and a unit elsewhere on the property a detached accessory dwelling unit (DADU)^[1]. Adding a unit within the house and building a detached one both take a construction addition or alteration permit there, while legalizing an existing unit takes a construction permit to establish use^[1].

Tiny houses sit on a line too. In Seattle, a tiny house on wheels is treated like a camper trailer and cannot be lived in on a lot within city limits, while a tiny house on a foundation is considered a DADU^[1]. Moab, Utah, lists three types: an external ADU, an internal ADU, and an internal ADU that requires an owner-occupied primary dwelling^[4].

Can you build one on your lot?

Each city decides where ADUs are allowed, and the rules are very different from one place to the next:

Private rules matter as well. Austin reminds applicants that some properties have deed restrictions or restrictive covenants, and that checking them is the applicant's responsibility^[2]. Moab's worksheet asks whether there is an HOA and recorded CC&Rs^[4].

What the application includes

An ADU permit is a full building permit. The Alameda County ADU Resource Center lists what applications typically include^[3]:

1. A site plan with the existing and proposed structures.
2. Architectural plans: floor plans, elevations and details.
3. Structural plans: foundation plan, framing plans and details.
4. Other items, such as Title 24 energy calculations in California, a deed restriction or an address assignment form.

Austin sets out the order in seven steps: confirm the lot qualifies, request a new address, apply for the building permit, pay the plan review fees and complete the review, pay and activate the permits, build with inspections, and receive the certificate of occupancy after the final inspection^[2]. In Moab, the path also includes a floodplain development application when the lot is in a floodplain, city impact fees and drainage impact fees, and recording the deed restriction during construction^[4].

What it costs in fees

ADU fees come from several bodies, not one. Alameda County lists planning, plan check and building permit fees, other department fees, school district fees for ADUs over 500 square feet, development impact fees for ADUs over 750 square feet, and utility and sanitation fees, and notes that many of the most expensive fees, like school and water fees, are set by other agencies^[3].

Moab, Utah, publishes an estimate for an external ADU^[4]:

Moab says actual permit fees vary by square footage and type of ADU^[4]. In Seattle, every ADU permit is reported to King County for sewer treatment capacity charges, and you sign a form acknowledging them with the application^[1]. Alameda County adds that some places require homeowners to upsize water or sewer service or meters^[3].

How long it takes

The Alameda County ADU Resource Center says the permitting phase typically takes 1 to 6 months, and that most ADU projects take 12 to 18 months from start to finish, some 24 months or more^[3]. Local staff review the application and provide comments within 60 days there, and must give a full set of comments with a list of incorrect items and how to fix them^[3].

Pre-approved plans are the fast lane where they exist. Seattle's ADUniverse site has a gallery of pre-approved detached ADU designs, and Seattle says that in most cases you can get the permit in 2 to 6 weeks with one^[1].

Once issued, keep the work moving. In Austin, an active permit expires on the 181st day if no inspection has been scheduled or received, and each inspection extends it by 180 days^[2].

Renting the ADU

The permit is also what lets you rent the unit legally. In Austin, a short-term rental license application needs the ADU's certificate of occupancy, and an ADU built after October 1, 2015, may not be used as a short-term rental for more than 30 days in a calendar year^[2]. In Moab, rental of external and internal ADUs is restricted to active employment households^[4].

An existing unit without a permit

Many ADUs already exist without one. Seattle has a dedicated path: legalizing an existing unit takes a construction permit to establish use^[1]. Legalizing has a cost beyond the permit itself: in New York City, legalizing unpermitted work on a one- or two-family home carries a $600 civil penalty, and removing the work instead carries the same penalty^[6].

How we handle ADUs

An ADU is a large project: it needs construction drawings, often structural plans and energy calculations, prepared by the licensed professionals your city requires. We are not architects or engineers. ADUs are therefore quoted case by case rather than sold at the All-in price. Describe your lot and your project on the quote form: we read your city's ADU rules for your address and tell you what we can prepare for the permit file, and what needs a licensed designer, before you commit to anything.

Common questions

Let us prepare your adu (accessory dwelling unit) permit

We research the rules that apply at your address and tell you exactly what your city needs before you commit to anything.

Get a quote for my project

Sources

1. City of Seattle, Construction and Inspections, Accessory Dwelling Unit. Checked October 13, 2026.
2. City of Austin, Texas, Development Services, Additional Dwelling Units. Checked October 13, 2026.
3. Alameda County, California, ADU Resource Center, Permit. Checked October 13, 2026.
4. City of Moab, Utah, ADU Process Flow Chart and Due Diligence Worksheet. Checked October 13, 2026.
5. City of Monticello, Minnesota, Building Safety Department, Basement Finishing Guide. Checked October 13, 2026.
6. New York City Department of Buildings, Form L2 instructions, civil penalties for work without a permit (rev. 03/2026). Checked October 13, 2026.

Keep reading

• Why ADU permits get rejectedThe real reasons an accessory dwelling unit permit is refused or stalls, from lot size and zoning to utilities, floodplains and deed restrictions, with official sources.
• Garage conversion permits, explainedDo you need a permit to convert a garage into living space? Yes. What cities require: drawings, energy code, egress, ceiling height, heat, parking and real fees.
• Basement finishing permits, explainedDo you need a permit to finish a basement? What cities ask for, the egress window, ceiling height and smoke alarm rules your plans must show, inspections and real fees.
• Permit Quote for ADUs, Garage Conversions and Tiny Houses
• Tiny house permits, explainedDo you need a permit for a tiny house? How the code's tiny house appendix works, why wheels change everything, and what zoning, utilities and inspections require.
• Barndominium permits, explainedDo you need a permit to build a barndominium? Why it is permitted as a house, when an engineer must stamp the plans, septic and floodplain permits, and real fees.