An accessory dwelling unit ordinance is the local law that tells you whether your lot qualifies for a second living space, how big and how tall that unit can be, where it can sit on the property, and what you have to do to get a permit and keep it legal after it’s built. Nearly 20 states now require cities and counties to allow ADUs in residential zones, but the specific construction and occupancy standards still come from your municipal code. Reading the ordinance carefully before you commission plans is what separates a project that gets built from one that stalls in review.
State Law and Local Ordinance: Which One Controls
ADU regulation runs on two tracks. State preemption laws in places like California, Oregon, Washington, and Colorado require cities to allow at least one ADU per single-family lot and cap what local governments can demand around size, parking, and owner occupancy. If your state has one of these laws, your city cannot ban ADUs in single-family zones outright.
Everything the state law leaves open falls to the local ordinance: the exact dimensional standards, the design rules, the permit process, and the fee schedule. Even in the strongest preemption states, dozens of details come from city hall. Your planning department is the authoritative source for what applies to your specific address, and calling them before you spend money on drawings is almost always worthwhile.
Does Your Lot Qualify
Most ordinances first authorize ADUs in single-family residential zones, then extend eligibility to multi-family and sometimes mixed-use zones. Minimum lot size is a common gate. Many codes require a lot of at least 4,500 to 6,000 square feet before a detached ADU is allowed, though several state laws now prohibit minimum lot size requirements altogether. Pull your zoning map and confirm your parcel’s designation before doing anything else.
Ordinances typically recognize three ADU types:
- Detached: a freestanding structure in the backyard, sometimes called a backyard cottage or casita.
- Attached: an addition to the primary home with its own entrance, kitchen, and bathroom.
- Conversion: an existing garage, basement, or attic repurposed into a self-contained unit.
Some jurisdictions also allow a Junior ADU, typically under 500 square feet, created inside the existing walls of the primary home or an attached garage. A property that qualifies for both a standard ADU and a Junior ADU is the main scenario where two accessory units are permitted on one lot. The type you pursue drives setback rules, permit complexity, and often the fee, so pick the category early.
Size, Height, and Setback Limits
The 2024 International Residential Code, which most jurisdictions adopt or adapt, caps ADUs at 1,200 square feet and no more than 50 percent of the primary dwelling’s floor area, with a minimum of 190 square feet.1International Code Council. 2024 IRC Appendix BC – Accessory Dwelling Units ADU Local codes often go tighter. Many municipalities cap detached ADUs at 800 to 1,000 square feet depending on lot size and zone.
Height caps for detached units generally sit between 16 and 25 feet, with the exact number tied to factors like distance from the rear property line and whether the unit has a second story. Some state preemption laws set a floor here. Washington, for example, prohibits cities from restricting ADU height below 24 feet.
Setbacks control how far the structure must sit from side and rear property lines. Four to five feet is the most common minimum for detached ADUs, but this varies. A conversion built inside an existing structure that already encroaches on the setback can sometimes keep its footprint without triggering a new setback. Get the numbers from the planning department in writing, because an error here can force a full redesign after plans are paid for.
Parking, Design, and Fire Safety Standards
Where parking is required, the usual standard is one off-street space per ADU. Exemptions are widespread. Most jurisdictions waive parking when the property is within a half-mile of public transit, several state preemption laws eliminate parking mandates altogether, and conversion ADUs that replace a garage frequently receive an exemption.
Architectural compatibility rules are common. Ordinances often require the ADU’s exterior siding, roof pitch, and window style to match or complement the primary home. Strictness varies significantly. Some cities enforce the rules through a design review committee; others simply require a note on the plans.
Fire safety requirements depend on whether the unit is attached or detached. Under the model building code, attached ADUs that share a wall or floor with the primary home need either a one-hour fire-rated separation between units or interconnected smoke and carbon monoxide alarms that activate in both units at once.1International Code Council. 2024 IRC Appendix BC – Accessory Dwelling Units ADU Some local codes require sprinklers in the ADU if the primary home is sprinklered. Detached units must have an egress path to the public way that does not pass through the main house.
Occupancy and Rental Rules After the Build
The ordinance keeps applying after construction. The most significant post-build rule is owner occupancy: many jurisdictions require the property owner to live in either the primary home or the ADU. Several states with preemption laws have eliminated or narrowed this requirement for standard ADUs, keeping it only for Junior ADUs or for short-term rental situations.
Short-term rental restrictions are also common. Many ordinances prohibit renting an ADU for stays under 30 consecutive days, aimed at keeping the unit in the long-term housing supply rather than on vacation rental platforms. Violations can trigger daily fines, and some jurisdictions can order removal of the unit if the owner fails to cure. Enforcement varies, so read the penalty section of your specific ordinance rather than assuming your city’s approach mirrors the next one over.
HOA Rules and Recorded Deed Restrictions
Zoning approval does not override private restrictions. If your property is in a homeowners association, read the CC&Rs before designing anything. A few states now prohibit HOAs from banning ADUs outright while still allowing reasonable design standards, but the protection is not universal. In states without it, HOA covenants can block the project regardless of what the ordinance allows.
On the public side, many ordinances require the owner to record a deed restriction against the property as a condition of permit issuance. The restriction typically states that the ADU cannot be sold separately from the primary home and may memorialize owner-occupancy or rental terms. It runs with the title, so future buyers inherit it, a title search reveals it, and any lender will see it. Treat this restriction as a permanent feature of the property.
The Permit Application and Review
The permit application is where the ordinance meets your project. Expect to submit a professional site plan showing the exact placement of all structures, property lines, easements, and existing utility connections, along with floor plans and elevation drawings that demonstrate compliance with the ordinance’s size, height, and setback limits. Incomplete or sloppy plans are the most common reason applications stall.
Beyond the drawings, most applications require:
- Utility connection details showing how the ADU will tie into water, sewer, and electrical service. Some jurisdictions require a separate water submeter when the ADU shares the primary home’s supply.
- Proof of ownership, usually a recorded deed or title report.
- Verification that your builder holds a valid license and appropriate bonding in your state.
- Soil or geotechnical reports in some areas, particularly for detached units with new foundations.
Assemble everything before submitting. A missing document can push review back by weeks.
Review timelines vary. Some jurisdictions with streamlined ADU programs approve straightforward applications in 30 to 60 days, while more complex projects or backlogged departments can take 120 days or more. State preemption laws sometimes impose deadlines on local review; if the city misses its statutory window, the permit may be deemed approved by default.
Fees add up quickly. Building permit fees typically range from a few hundred dollars to several thousand, depending on project valuation. On top of that, some jurisdictions charge development impact fees to offset the ADU’s burden on schools, parks, and water systems. Impact fees can run from roughly $2,000 to over $10,000 in high-cost areas, though some state laws exempt smaller ADUs from impact fees entirely or cap them at a fraction of what a new single-family home would pay.
After plans are approved, construction proceeds through inspections at foundation, framing, electrical and plumbing rough-in, insulation, and final. Each must pass before the next phase begins. After the final inspection, the city issues a Certificate of Occupancy, which authorizes you to use the unit as a residence. You cannot legally rent or occupy the ADU without it.
What Happens If You Build Without a Permit
Skipping the ordinance is one of the most expensive shortcuts a homeowner can take. Code enforcement can issue daily fines and ultimately order an unpermitted structure removed at the owner’s expense. Homeowners insurance may deny a claim involving an unpermitted structure. When you sell, the unit surfaces during the buyer’s inspection or title search, and it can kill the deal or force a steep price reduction.
Lenders have grown more sophisticated. Fannie Mae’s appraisal guidelines distinguish between permitted and unpermitted units, and an unpermitted ADU can complicate refinancing or reduce the appraised value of the property. Some jurisdictions offer amnesty or legalization programs for existing unpermitted units, but these usually require bringing the structure into full code compliance, which can cost about what doing it right the first time would have.