Do I Need a Permit for an ADU?

An accessory dwelling unit, or ADU, is a second, smaller home on the same lot as a single-family house. It may be a detached backyard cottage, an addition attached to the main house, a converted garage, or a basement or attic apartment. Some cities also allow a junior ADU carved out of existing space in the main house. Because an ADU is a complete place to live, with sleeping, cooking, and bathroom facilities, it always needs permits.

ADU permitting has two layers. Zoning decides whether you can build an ADU on your lot and how big, tall, and close to property lines it can be. The building code decides how it must be built. Both are administered locally, and both vary widely, so start by contacting your local building department and planning or zoning office.

Why ADUs always need permits

Small accessory buildings are sometimes exempt. The 2021 International Residential Code (IRC) exempts one-story detached accessory structures up to 200 square feet of floor area (R105.2, Building item 1), and many cities use a smaller limit. That exemption is meant for sheds and similar buildings. The IRC defines a dwelling unit as “a single unit providing complete independent living facilities for one or more persons, including permanent provisions for living, sleeping, eating, cooking and sanitation” (Chapter 2). Once a structure meets that definition it is a dwelling, not an accessory structure, and it needs a building permit regardless of size. Converting an existing garage or basement also needs one, because IRC R105.1 requires a permit to change the occupancy of a building or to install or alter its electrical, gas, mechanical, or plumbing systems. An ADU typically requires:

• Zoning or planning approval confirming the ADU is allowed and meets size, setback, height, and parking rules.
• A building permit covering foundation, framing, insulation, fire safety, and egress.
• Electrical, plumbing, and mechanical permits, either bundled with the building permit or issued separately.
• Utility approvals for water, sewer or septic, and electric service.
• A final inspection and approval for occupancy before anyone lives there. See the certificate of occupancy guide.

Zoning comes first

Before paying for full construction drawings, find out what your zoning allows. Questions to ask your planning department:

• Are ADUs allowed in my zoning district, and are detached, attached, and conversion ADUs all permitted?
• What is the maximum size, often expressed as square feet or a percentage of the main house?
• What are the setbacks from side and rear property lines, and the height limit?
• Is additional off-street parking required?
• Is there an owner-occupancy requirement?
• Are short-term rentals allowed?
• Does a historic district, overlay zone, or HOA covenant add restrictions?

State law matters here. A growing number of states, including California, Oregon, and Washington, have passed laws requiring cities to allow ADUs and limiting the restrictions cities can impose (examples below). Other states leave ADU rules entirely to local governments, and some cities still prohibit them. If your state has an ADU law, its housing or community development agency usually publishes guidance.

HOA rules are separate from city rules. In some states, ADU laws limit how far HOAs can restrict ADUs, but where they do not, deed restrictions may prohibit one even if the city allows it.

Building code requirements

An ADU is built to the same residential code as any dwelling. Key items, with 2021 IRC sections:

• Foundation and structure designed for local snow, wind, and seismic loads.
• Emergency escape and rescue openings in every sleeping room, and in any basement (R310.1). See the basement guide for opening sizes.
• Minimum ceiling heights of 7 feet for habitable space and hallways, and 6 feet 8 inches for bathrooms and laundry rooms (R305.1).
• Smoke and carbon monoxide alarms meeting new-construction requirements (R314, R315).
• Energy code compliance, including insulation, windows, and air sealing (Chapter 11).
• Fire separation from the property line. Fire separation distance is measured to the nearest interior lot line, or to an imaginary line between two buildings on the same lot (Chapter 2 definition). Under Table R302.1(1), an exterior wall closer than 5 feet needs a 1-hour fire-resistance rating, openings are not allowed closer than 3 feet, and openings between 3 and 5 feet are limited to 25 percent of the wall area. Sprinklered dwellings use the more lenient Table R302.1(2).
• Fire separation from the main house. An attached or internal ADU turns the building into a two-family dwelling, and R302.3 requires 1-hour fire-resistance-rated walls and floors between the units, reduced to 1/2 hour if the building is sprinklered throughout.
• Sprinklers. Model IRC R313.2 requires sprinklers in new one- and two-family dwellings but exempts additions and alterations to existing unsprinklered buildings. Whether your state kept R313.2 decides whether a new detached ADU needs sprinklers, so ask.
• Independent heating and ventilation, kitchen and bathroom exhaust, and safe stairs and guards.

Garage and basement conversions are where these rules bite. Garages often lack insulation, adequate foundations, and egress, and their slabs may be too low or sloped. Basements may lack ceiling height and egress windows. See the basement guide for model egress dimensions.

How the rules differ by state and city

State ADU laws and local programs change the zoning answer, the fees, and the timeline far more than the building code does. Four examples, checked against official sources in October 2026:

• California (statewide). The Department of Housing and Community Development’s ADU Handbook (January 2025, with a January 2026 addendum) summarizes Government Code sections 66310-66342. Cities must approve or deny a complete ADU application ministerially within 60 days, or it is deemed approved (66317). Under SB 543, effective January 1, 2026, the agency must also tell you within 15 business days whether the application is complete. A local ordinance cannot block an 800-square-foot ADU with 4-foot side and rear setbacks (66321(b)(3)), and detached ADUs get at least a 16-foot height limit (18 feet near major transit). No impact fees apply to an ADU with 750 square feet or less of interior livable space; larger ADUs pay fees proportional to the main house. Cities cannot impose owner-occupancy on an ADU (66315). A junior ADU (JADU) can be no more than 500 square feet inside the main house; under AB 1154, effective 2026, the owner must live on site only if the JADU shares sanitation facilities with the house, and JADUs cannot be short-term rentals. Building an ADU cannot trigger sprinklers in the existing house (66314(d)(12)). Find your local office for Los Angeles, San Diego, San Jose, or Sacramento.
• Washington (statewide). RCW 36.70A.681, from the 2023 law HB 1337, requires covered cities and counties to allow at least two ADUs on lots in urban growth areas that allow single-family homes. They cannot require owner-occupancy, set a maximum size below 1,000 square feet or a roof height limit below 24 feet (unless the main house’s limit is lower), charge impact fees above 50 percent of those for the main house, or require off-street parking within a half-mile walk of a major transit stop. Garages and other existing structures can be converted even if they violate current setback or lot coverage rules.
• Seattle. Seattle allows two ADUs per lot, an attached ADU (AADU) and a detached one (DADU), and does not require parking for them. Adding a unit in the house or building a detached one takes a construction addition/alteration permit; legalizing an existing unit takes a construction permit to establish use. The city’s ADUniverse site offers pre-approved DADU plans, and Seattle says most of those permits issue in 2 to 6 weeks. King County bills a sewage treatment capacity charge for each new ADU, and you sign an acknowledgment form when you apply. Tiny houses on wheels cannot be lived in on Seattle lots; on a foundation, they are DADUs.
• Austin. Austin counts a structure as a dwelling unit if it has habitable space, a full bathroom, and permanent cooking facilities, which triggers zoning, utility meter, and address rules. You must get a unique address or building number for the ADU before you apply for the building permit. Fire separation follows IRC Table R302.1(1). An ADU built after October 1, 2015 may be used as a short-term rental for no more than 30 days a year, and the rental license application needs the ADU’s certificate of occupancy.

Legalizing an ADU built without a permit

Many older houses already have a basement or garage apartment that was never permitted. Seattle handles it with a construction permit to establish use. California bars cities from denying a permit for an unpermitted ADU built before January 1, 2018 just because it violates building standards or ADU rules, unless correcting the violation is needed to protect health and safety or the building is substandard (Government Code 66332). Expect the inspector to open walls where needed to see wiring, plumbing, and framing. The unpermitted work guide covers the process and the risks of leaving it alone.

Utilities and site work

Utilities often drive ADU cost and schedule as much as construction does:

• Sewer or septic. A connection to the public sewer may require a new lateral or a capacity review. On septic, the system usually must be evaluated for added bedrooms, often by the county health department.
• Water. You may share the main house’s service or need a separate meter.
• Electric. The existing service may need an upgrade, and a separate meter may be optional or required.
• Address. Many jurisdictions assign the ADU its own address for emergency response.

Call 811 before any excavation so underground utilities are marked. If the lot is in a mapped floodplain, floodplain rules apply to the new dwelling.

What the inspection covers

ADU inspections follow the sequence for a new house or addition:

1. Site and setback verification, and underground utilities.
2. Foundation and footing inspection before concrete is placed.
3. Underfloor or slab inspection, including under-slab plumbing.
4. Framing, rough electrical, plumbing, and mechanical inspections.
5. Insulation inspection.
6. Drywall or fire-rated assembly inspection where required.
7. Final building, trade, and zoning inspections, after which the ADU can be occupied.

The building inspection stages guide covers scheduling and preparation.

What to have ready when you apply

Expect a fuller application than for most residential projects:

• A site plan showing property lines, the main house, the ADU, setbacks, easements, parking, and utility routes.
• Floor plans, elevations, sections, and construction details.
• Structural calculations where required, especially in high wind or seismic areas.
• Energy code compliance documentation.
• Utility and sewer or septic information.
• Contractor licenses, or an owner-builder declaration if allowed.
• A new address or unit number for the ADU, if your city assigns one before permitting, as Austin does.
• Utility forms, such as a sewer capacity charge acknowledgment where your sewer agency bills one, as King County does in Seattle.

Some cities offer pre-approved ADU plans that shorten review. Fees often include plan review, building and trade permits, and utility connection or impact fees. The building permit cost guide explains how fees are calculated, and how to apply for a building permit walks through the process.

Hiring a contractor vs doing it yourself

ADUs are full residential construction. Many jurisdictions allow owner-builders to permit an ADU on their own property, but the rules on selling or renting owner-built structures soon after completion vary by state, and licensed trades may be required for electrical, plumbing, or gas work. Read the owner-builder permits guide before deciding.

Practical tips

• Get zoning answers in writing before paying for full drawings.
• Ask whether your city has pre-approved plans or a dedicated ADU review track.
• Get utility connection estimates early, as they can change the budget significantly.
• Confirm rental and occupancy rules before you commit if rental income is the goal.