How to Build an ADU: A Step-by-Step Guide

Step 1: Check feasibility and your local ADU rules

Before you spend a dollar on plans, confirm that an ADU is actually allowed on your property and learn the rules that will shape it. There is no single national ADU code: regulations are set primarily at the state and local level, so what's permitted in one city may be restricted — or far easier — in the next town over. Start by identifying your zoning and the local limits on ADUs: maximum size, height caps, required setbacks from property lines, parking requirements, and how many units your lot can have.

State law matters too. Some states, California most prominently, have passed legislation 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 laws like SB-9 open up lot splits and small-scale infill on many single-family parcels. Others leave more discretion to individual municipalities. You can explore permit activity and coverage state by state on our state coverage hub, including a detailed look at California, where ADU-friendly rules have driven some of the highest activity in the country. The most reliable source for your specific rules is your city or county planning department — call them or read their ADU handout early, because the answers here determine everything that follows. Treat all of this as background, and verify the current rules where you actually plan to build.

Step 2: Set a budget and line up financing

Once you know an ADU is feasible, the next step is money — both what the project will cost and how you'll pay for it. ADU costs vary enormously by type, size, location, site conditions, finishes, and local fees, so build your budget around your specific project rather than a headline number. Our companion guide on how much an ADU costs breaks down the major cost drivers, typical ranges by type, and the fees first-time builders often forget. Remember to budget for more than construction: design and engineering, the building permit itself, plan-review fees tied to your project's valuation, possible impact fees, and utility-connection charges are all real line items.

With a realistic budget in hand, plan how you'll fund it. Few homeowners pay entirely out of pocket, so most projects rely on some form of financing — home equity, construction or renovation loans, or dedicated ADU loan products. Our guide on how to finance an ADU walks through the main options and how to match one to your project. Lining up financing early matters because some lenders underwrite against the value or income the finished unit will add, and that conversation can take time. None of this is financial advice — confirm current costs with licensed contractors and current loan terms with a licensed lender.

Step 3: Design your ADU and choose a type

With feasibility and budget settled, you can design the unit — and the first design decision is the type, because it drives cost, timeline, and which rules apply. A detached ADU is a standalone structure in the backyard; an attached ADU shares a wall with the main house; a garage, basement, or attic conversion turns existing space 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. Conversions tend to be the least expensive because the shell already exists, while detached new construction usually costs the most but offers the most flexibility. For a closer look at the trade-offs, see detached vs attached ADUs.

Let your goals guide the layout: who will live there, whether you want one bedroom or two, and how the unit relates to the main house and yard. Match the design to the local rules you gathered in step one — size caps, setbacks, and height limits — so you don't design something the city won't approve. For anything beyond the simplest conversion, you'll typically work with a designer or architect to produce construction drawings and a site plan; whether one is legally required depends on your jurisdiction and the project's complexity. A clean, code-aware design at this stage is the single best way to avoid expensive revisions during plan review.

Step 4: Navigate the permit process

With drawings in hand, you submit for a permit. Because an ADU creates a new dwelling unit, it requires a building permit almost everywhere, the same way new construction does. You'll file your application — increasingly through an online portal — along with your construction drawings and site plan, and pay submission and plan-review fees. The application then enters plan review, where staff check your submittal against building, electrical, plumbing, mechanical, and zoning codes and may return correction requests, often called plan-check comments.

This back-and-forth is normal. You address the comments, resubmit, and repeat until the plans are approved — and 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 you can legally start construction. The full mechanics of this stage, including how fees and timelines tend to work, are covered in depth in our complete ADU permit guide. Keep your approved plans and issued permit on site once work begins; you'll need them for the inspections that come next, and building without an issued permit can lead to stop-work orders, fines, and costly do-overs.

Step 5: Build it and pass final inspection

With the permit issued, construction begins — and the permit process continues right through the build in the form of inspections. As work reaches key milestones, a city inspector verifies that what was built matches the approved plans and meets code. Typical inspection points include the foundation, the framing, the rough-in of the electrical, plumbing, and mechanical systems before walls are closed, and a final inspection once everything is complete. Each must pass before the next phase proceeds, so coordinating inspections with your contractor's schedule keeps the project moving.

Whether you hire a general contractor or act as your own — see owner-builder — staying organized through this phase pays off. After the final inspection passes, the jurisdiction issues a certificate of occupancy (or equivalent sign-off), the official confirmation that the unit is safe and legal to live in. Only then is your ADU truly finished in the eyes of the city. Don't skip or shortcut inspections to save time: an unpermitted or un-inspected unit can be impossible to rent or refinance legally and can create serious problems when you eventually sell.

Step 6: Track permit activity (optional)

Throughout an ADU project — and especially if you're a builder, lender, designer, or supplier who serves ADU owners — it helps to see where activity is happening. Every ADU permit is a public record of real demand: it documents that an owner has committed time and money to building a unit. The catch 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 65 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 fastest way to see what this looks like is our free permit lookup, where you can search recent activity in a market and gauge how busy the ADU scene is near you. 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. As always, this is general information, not legal or construction advice; confirm every requirement with your local jurisdiction.

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