Building an ADU in Austin: what to settle first

Before you hire anyone to build an ADU in Austin, find out whether your lot can have one. Everything else, the design, the budget, the builder, depends on that answer, and the person who can settle it is the city's permit office, not a website. An accessory dwelling unit might be a backyard cottage, a garage conversion, a unit over a garage or an attached suite. Each of those has its own problems, but they all start with the same lot questions.

Our ADU construction page describes how we handle these projects. What follows is what you can do before that first call.

Check the lot before you draw anything

Most people start with a floor plan. Start with the property instead.

Whether an ADU is allowed, and in what form, depends on zoning, the size and shape of the lot, where the existing house sits, and the rules of whichever city the property is in. Those rules change over time, and they're different in the City of Austin than in Round Rock, Cedar Park, Lakeway or Georgetown. Any general statement about what's allowed, including this one, is a starting point. Get the answer from the city or from a written zoning check, and get it before you pay for drawings.

Several things on the lot tend to decide it: where the house, driveway and easements are, flood and watershed rules that affect some properties, protected trees that limit where a structure can go, and whether there's room to get to a second dwelling at all.

Deed restrictions and HOA review

A city can allow an ADU that your deed or neighborhood association doesn't. Plenty of Austin-area neighborhoods have restrictions or an HOA with architectural review, and those can bar a second dwelling, limit exterior materials, or require design approval before anything gets built.

Read your restrictions early. If the association has to approve the project, find out what they want to see and how long they usually take. City permits and HOA approval run separately and both have to clear. Starting the HOA conversation after the drawings are done is one of the more avoidable delays on a backyard project.

Utilities and access

A second dwelling needs water, sewer and power, and usually gas or something in place of it. The real question is how each one gets from where it is now to the new building.

Find out where the water and sewer lines run and whether they can serve another unit or need a new connection. Look at the electrical service and the panel at the main house, because a second unit may call for a bigger panel, a subpanel or its own meter. Ask whether separate metering is required or allowed, and whether it changes how the unit can be used.

Distance adds up fast. A cottage at the back of a long lot needs longer runs than one beside the house, and a trench across a yard, under a driveway or around mature roots is work that doesn't appear on any floor plan.

Access matters twice. During construction, it decides how materials and equipment reach the site, and a narrow side yard costs real money. After construction, it decides how people get to the unit, including the path, lighting and any emergency access the rules require.

Soil, foundation and drainage

East of I-35 the soil is expansive clay that swells and shrinks with moisture, and the foundation has to be designed for that. West of the interstate, shallow limestone changes how excavation, footings and drainage work. The same unit costs different amounts depending on what's under it.

Drainage gets its own mention because a new roof and slab change where rainwater goes. In a backyard, the new runoff can end up at the main house or the neighbor's fence if nobody planned the grading. Clay and limestone site conditions in Austin explains what to ask about this.

Permits and the order of work

An ADU normally goes through design, permit review, construction and inspections, with some overlap. Unresolved lot questions come out during permit review, which is why the zoning check comes first.

1. Confirm what the lot and the neighborhood allow.
2. Survey the site and the utilities.
3. Finish the design and put the scope in writing.
4. Submit for permits and answer review comments.
5. Build, with inspections at the stages the city requires.
6. Pass the final inspection and get utilities turned on.

Some material orders and site prep can happen during review, depending on what the city allows. Starting construction before approval is a risk you carry alone. For what to expect from permit offices, see Austin building permits for remodels and additions.

What the unit will be used for

Decide the use before you design. A guest suite, a rental, a place for a parent and a home office each want a different layout, kitchen, bathroom, parking arrangement and entrance.

Rules about occupancy, short-term rental, owner residency and separate utility service differ by lot and by city, and they can change after you build. Get your intended use confirmed by the city and keep that confirmation with your project records. If your numbers only work when the unit is rented, learn that before the foundation is poured.

Why ADU bids don't match

ADU bids spread out more than most, and the reason is usually what's in them. One bid includes site utilities, trenching, a new meter, grading and the permit work. Another starts at the slab and leaves the rest to you. One carries real selections for cabinets, fixtures and flooring, and another carries allowances that look small because they assume small choices.

Ask every builder the same questions. What utility work is included? Who handles permits and HOA submittals? What does each allowance cover? What's excluded? What does the bid assume about soil, trees and access? How to compare renovation bids shows how to line proposals up. A bid with nothing about site work isn't cheaper. It left out something you'll pay for anyway.

Bring the address, any survey you have, your deed restrictions or HOA documents, what you want the unit to do, and a rough idea of size and finish. A builder can tell you what the site raises, and you can tell them what the unit needs to do.

If you're weighing an ADU against an addition to the main house, the home additions page covers that, and the Austin page lists where we work. To talk through a specific lot, request an estimate.