The Complete ADU Permit Guide (2026)

What an ADU permit is

An ADU permit is the building department's authorization to construct an accessory dwelling unit — a smaller, independent home with its own kitchen and bathroom on the same parcel as an existing house. Because an ADU creates a new dwelling unit, it requires a building permit virtually everywhere, the same way new construction does. The permit confirms that the design meets adopted building, electrical, plumbing, mechanical, and zoning codes before work begins and through final inspection.

ADUs come in several forms, and the permit pathway can differ by type. A detached ADU is a standalone backyard structure; an attached ADU shares a wall with the main house; a conversion turns existing space such as a garage or basement into a unit; and a junior ADU (JADU) is a small unit carved out of the existing footprint of the primary home, often a converted bedroom. Each type touches zoning, setbacks, parking, and utility rules differently, which is why two ADUs on the same block can face very different approval steps.

The ADU permit process, step by step

While every jurisdiction has its own quirks, the ADU permit process generally follows a consistent arc. It starts with research: confirm that an ADU is allowed on your lot and learn the local size limits, setbacks, height caps, and parking rules. Many cities have adopted streamlined or ministerial ADU paths, but you need to know which one applies to your project before you design.

Next comes design and document preparation. You'll typically need construction drawings, a site plan, and details on structural elements and the electrical, plumbing, and mechanical systems. For anything beyond the simplest conversion, stamped plans from a licensed design professional are often required. You then submit the application to the local building or planning department — increasingly through an online portal — and pay submission and plan-review fees.

The application enters plan review, where staff check your submittal against code and may return correction requests (plan-check comments). You address those and resubmit; responding quickly and completely is the single biggest lever you control over the timeline. Once plans are approved and fees are paid, the permit is issued and construction can begin. During the build, inspections verify the work at key stages — foundation, framing, rough systems, and a final inspection — before the unit can be occupied. For the broader mechanics of this process, see our contractor's guide to building permits.

ADU costs and timelines (general ranges only)

Cost and timeline for an ADU vary enormously by type, size, location, finishes, site conditions, and local fees, so treat any figure as a broad range rather than a quote. In general, conversions of existing space — like a garage conversion — tend to be the least expensive because the shell already exists, while detached new-construction units typically cost the most. Utility connections, foundation work, and impact or development fees can move the total significantly in either direction.

Timelines follow a similar pattern. Design and permitting can take anywhere from a couple of months to considerably longer depending on how the jurisdiction reviews ADUs, and construction adds several more months on top. Where a city offers streamlined or ministerial review, the approval stage can be meaningfully shorter. Permit fees themselves are often tied to the project's valuation — the estimated cost of construction — plus separate plan-review and sometimes impact fees, and each jurisdiction sets its own schedule. The honest takeaway is that ADU costs and timelines are highly jurisdiction-dependent; always price against your specific city, lot, and design, and confirm fees with the building department. This is general information, not financial or legal advice.

ADU rules vary by state and city

There is no single national ADU rulebook. ADU regulations 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, by-right approval paths for qualifying projects. Related California laws like SB-9 open up lot splits and small-scale infill on many single-family parcels. Other states leave far more discretion to individual municipalities, so what's permitted in one city may be prohibited in the next town over.

Because of this variation, the most important step before you design anything is to check the rules where you actually plan to build. You can explore permit activity and coverage state by state on our state coverage hub, including a detailed look at California, where ADU-friendly legislation has driven some of the highest activity in the country. For more on the California laws that interact with ADU development, read our overview of SB-9 and SB-10 explained. Treat all of this as background — verify current rules with your jurisdiction.

Tracking ADU permit activity

Once you understand the permit process, the next question for builders, lenders, and suppliers is where ADU activity is actually happening. Every ADU permit is a public record of real demand: it tells you, in the open record, that a property owner has committed time and money to building a unit. The problem is that this signal is scattered across hundreds of city and county systems in different formats, and it goes stale quickly.

This is the gap Igni fills. Igni ingests permit records directly from official municipal open-data portals across 60 cities in 37 US states with sub-24-hour freshness, normalizes them into one consistent format, and applies a typed ADU classification — detached, attached, junior ADU, conversion, or unknown — inferred from each jurisdiction's structured flags and work descriptions. For California, records also carry SB-9 and SB-10 flags so you can spot eligible activity early. The result turns scattered public records into a clean, current view of ADU demand in your markets. Igni is contact-driven during its pilot — there's no public self-serve checkout yet — so to see coverage in your area and get access, request access.

Frequently asked questions