ADU permits and zoning: what to check before you build

An accessory dwelling unit (ADU) is a second, smaller home on the same lot as a house: a detached backyard cottage, a unit above or in the garage, a basement apartment or an attached addition. Whether you can build one, and how big, depends first on local rules. A few checks before you hire a designer save months.

1. Is an ADU allowed on your lot?

Zoning is set by your city or county. Many cities now allow ADUs in single-family zones, and several states, California among them, have passed laws that limit what local governments can forbid. Rules still vary a lot from one place to the next. Look for your city's ADU page or call the planning department and ask:

• whether ADUs are allowed in your zone, and how many per lot;
• the maximum size and height;
• setbacks from property lines and the main house;
• parking requirements, if any;
• whether the owner must live on site, and whether short-term rentals are allowed.

If you live in a community with an HOA, read its covenants too.

2. Utilities and site constraints

An ADU needs water, sewer or septic, and electricity. Ask whether it can share the existing connections or needs new ones, and whether the electrical panel has enough capacity. On a septic system, the system may need to be evaluated or upgraded for the extra bedrooms. Slopes, easements, large trees and flood zones can also limit where the unit goes.

3. Design and plans

The unit must meet the building code for dwellings (structure, fire separation, egress, insulation and energy rules). Plans are drawn by a designer or an architect; structural calculations are often required. Some cities offer pre-approved ADU plans that can shorten the review.

4. The permit process

1. Submit the plans to the building department, with the site plan and any zoning forms.
2. Pay the plan review and permit fees. Some jurisdictions also charge impact or connection fees for utilities.
3. Answer the plan reviewer's comments and receive the permit.
4. Build, with inspections at each stage (foundation, framing, electrical, plumbing, insulation, final).
5. Get the final approval or certificate of occupancy before anyone moves in.

5. Choose a builder who has done it before

ADUs combine small-house design with tight lot constraints. Ask builders for ADUs they completed in your city, who handles the permit, and what their contract includes (design, permit fees, utility connections, site work).

Describe your lot and your project in the form: local ADU builders and designers get back to you with a quote.