ADU Permit Requirements: What Homeowners Need to Know

What Counts as an ADU

ADUs come in two main forms. An attached ADU (Seattle calls it an AADU) sits inside or is attached to the main house, such as a basement or garage conversion. A detached ADU (DADU), often called a backyard cottage, is a separate building.

Cities define a dwelling unit carefully because that definition triggers the extra rules. Austin describes a dwelling unit as habitable space with a full bathroom and permanent cooking facilities. That is why a home office with a half bath is usually just an accessory building, while the same space with a kitchen becomes a dwelling unit with its own requirements.

Start with Zoning

Before any design work, confirm that your lot allows an ADU and how large it can be. Examples from official sources:

• Austin ties ADUs to single-family zoning districts (SF-1, SF-2 and SF-3) and a minimum lot size of 5,750 sq ft, and requires each unit to have a unique address.
• Seattle allows ADUs under SMC 23.42.022 and says that only two of the units on a lot may be ADUs. Its Tips 116A and 116B cover attached and detached units.
• California has statewide ADU laws that limit what local governments can prohibit; the state housing department (HCD) publishes guidance and a handbook.

Zoning rules commonly address maximum size, height, setbacks, lot coverage, parking and whether the owner must live on site. They differ by city and change often, so read the current version.

Building Code Requirements

An ADU must meet current building codes for a dwelling; Seattle notes this applies even when converting existing space. Common items include:

• Fire separation. IRC R302.1 sets fire-resistance rules for exterior walls close to property lines, which affects where a detached unit can sit and what its walls are made of. Austin cites R302.1(1).
• Egress and ceiling height for sleeping rooms, which can rule out some basements.
• Smoke and carbon monoxide alarms.
• Separate or upgraded utilities: electrical service capacity, water and sewer connections.
• Energy code insulation and window requirements.

Fees and Utility Charges

ADU permit fees vary by jurisdiction and are usually based on valuation or floor area, so we do not quote a typical figure. Expect more than just the building permit: plan review fees, utility connection charges and, in some places, impact fees. Seattle, for example, notes that a new ADU may owe a King County sewer capacity charge. Ask your permit office and utility for a fee estimate before you commit to a design. For how fees are structured in general, see How Much Does a Building Permit Cost? Verified Fee Examples.

The ADU Permit Process Step by Step

1. Feasibility. Confirm zoning, lot size, setbacks, utilities and any private restrictions.
2. Pre-application advice. Many cities offer coaching or pre-submittal meetings; Seattle offers free 20-minute coaching.
3. Design. Use pre-approved plans where available. Seattle offers pre-approved DADU plans through its ADUniverse program.
4. Submit construction documents under the local version of IRC R106 and respond to review comments.
5. Permit issue and construction, with inspections at each stage (see Building Permit Inspections: What Inspectors Check and When).
6. Final approval and, where required, a certificate of occupancy under IRC R110, plus a unique address.

Legalizing an Existing Unit

Many homes already have an apartment that was never permitted. Seattle states that such a unit is not legal until established through a permit and that legalizing it requires a construction permit to establish the use. The unit has to meet current code, which can mean upgrades to fire separation, egress or ventilation. If you are buying or selling a home with an unpermitted unit, read Selling a House with Unpermitted Work: Risks and Options.

Questions to Answer Before You Hire a Designer

• Which zone is my lot in, and does it allow an attached or detached ADU?
• What are the maximum size, height and setback limits?
• Does my lot meet any minimum lot size, as Austin's does?
• Is there enough electrical service capacity, and where would water and sewer connect?
• Are there pre-approved plans I can use?
• Do deed restrictions or HOA rules limit a second unit?
• Will the unit need its own address, and how is that assigned?

Write the answers down with the source for each. They become the brief for your designer and reduce the risk of a redesign after plan review comments. If you plan to rent the unit, also ask about any rental registration or licensing your city requires; those rules sit outside the building permit.

Building Departments in the Cities We Cover

Rules in this guide vary by jurisdiction. These are the official departments that issue residential permits in the ten cities we track, with the city's open-data source for permit records. Each city page shows the rules for roofing, HVAC, solar, fence and deck work.

Before relying on any rule, read the department's current page or call its permit desk. Model-code sections cited here (such as IRC R105) are adopted locally, often with amendments.

Frequently Asked Questions