How to build an ADU in California, from lot check to move-in

Steps to build an ADU in California

The construction itself is the part people picture, but most of the expensive mistakes happen before it starts. This table is the map. Each row is covered below, and the sibling guides go deeper on the construction phases and the calendar.

For what happens on site from grading to punch list, read the ADU construction process. For how long each stage takes, see how long it takes to build an ADU. If you're still deciding whether you want one at all, start with what an ADU is.

You won't be breaking new ground. California permitted 30,354 ADUs in 2024, more than a quarter of all homes permitted in the state that year, so cities and builders have well-worn processes.

Check what your lot allows

Feasibility answers three questions: where a unit can sit, how big it can be, and what it will take to bring water, sewer and power to it. Most of the answer comes from state law, then your city's code fills in the details.

What California guarantees

• A by-right approval. ADUs are approved ministerially, without a hearing. On a single-family lot, state law allows a converted ADU, a detached ADU of up to 800 sq ft with 4-foot side and rear setbacks, and a junior ADU inside the house.
• No owner-occupancy rule for an ADU, so you can rent either the unit or the main house.
• Limited parking rules. No parking can be required within half a mile walking distance of public transit, for a unit that's part of the house or an existing structure, or within a block of a car-share vehicle.
• HOAs can't block it. State law voids CC&Rs that prohibit or unreasonably restrict an ADU on a single-family lot. An HOA may set reasonable design standards, but it can't charge fees for approval.

What San Diego adds

In the City of San Diego, the municipal code allows a new ADU from 150 to 1,200 sq ft, with no maximum for conversions of existing space. An ADU 16 feet tall or less needs no interior side or rear setback, except 4 feet in high and very high fire hazard zones. ADUs can be two stories at most in single-family zones, and parking is required only in the Beach Impact Area outside transit priority areas.

Look up your zone in the city's Zoning and Parcel Information Portal (ZAPP), as the city's ADU bulletin suggests. The same bulletin flags the overlays that change a project: lots in the Coastal Overlay Zone usually need a Coastal Development Permit, and fire hazard zones bring extra setbacks.

Then walk the yard with the utilities in mind. How far is the unit from the sewer line, and is it uphill or downhill? Where is the electrical panel? A long trench or a sewer pump changes the budget more than most design choices do. Our ADU utilities guide covers meters, sewer and panel upgrades, and California ADU laws has the full state rulebook.

Choose the kind of ADU

The type you choose sets most of the cost and much of the permit path. Converting space you already have is cheapest per unit; a new detached building gives you the most privacy and rent.

Those are Lars Remodel's published planning ranges, a builder's figures rather than quotes. For the trade-offs between types, see attached vs. detached ADUs, the garage conversion ADU guide, junior ADU rules and basement and attic ADUs. If space is tight, a small ADU saves fees, and ADU in the backyard covers placement and access.

Budget for building an ADU

Set a total budget before you talk to designers, and include everything: design, fees, sitework and a contingency. The most reliable public San Diego numbers come from builders who publish their prices.

Snap ADU's published prices cover design, permits, basic sitework and utilities, but list extra grading, retaining walls, demolition, added utility work and permit, school and sewer fees as variables on top. Three budget items catch people:

• City fees. Snap ADU's fee table estimates City of San Diego fees at $10,734 for a 499 sq ft unit and $31,111 for 1,199 sq ft (February 2026). That's a builder's estimate, not the city's schedule. The city exempts the first two ADUs on a lot from development impact fees.
• Sitework. Snap ADU says $25,000 covers sitework on a flat lot when tie-ins stay on your property. A separate SDG&E meter runs about $10,500, a sewer pump about $5,000, and long trenches up to $200 a linear foot.
• Property tax. The County Assessor adds the value of the new construction only; your existing house isn't reassessed.

Most homeowners borrow. The common routes are a HELOC, a home equity loan, a cash-out refinance, a construction loan, or a renovation mortgage such as Fannie Mae HomeStyle Renovation or Freddie Mac CHOICERenovation. See ADU financing and ADU grants in California, run your numbers in the ADU cost calculator, and read what an ADU costs in San Diego for the full breakdown. If the unit will be a rental, ADU rental income in San Diego shows the payback math.

Pick who designs and builds it

This is the decision that shapes the rest of the project. There are four common routes:

You don't always need an architect. California lets anyone prepare plans for wood-frame dwellings of up to two stories and a basement, which covers most ADUs. Engineering is a separate question; your plans will still need structural and energy work. Compare the routes in design-build vs. architect, see ADU architects and designers, and read prefab ADUs in California.

Hiring the builder

Whichever route you take, the builder needs the right license. The Contractors State License Board says a B (General Building) contractor can build an ADU, a C-47 contractor can install a manufactured-home ADU, and specialty trades like electricians and plumbers can't run the whole job. It treats an ADU as a home improvement project, so you need a written contract, and you can cancel within three days of signing (five if you're 65 or older). The board also recommends getting at least three bids.

The deposit is capped by law at $1,000 or 10% of the contract price, whichever is less. A builder asking for more up front is a reason to walk away. Compare verified ADU builders in San Diego, check a contractor's license, and read how to hire an ADU contractor before you sign.

Design and plans

Design turns the feasibility answers into a permit-ready plan set. Expect a floor plan, elevations, a site plan, structural drawings and Title 24 energy calculations. The City of San Diego also asks for forms such as a Water Meter Data Card with the application.

Decisions you'll make here:

• Layout and size. Bedrooms, bathroom count and whether you stay under the fee lines at 500 and 750 sq ft. See ADU floor plans and ADU design ideas.
• Kitchen and bath. They're the most expensive rooms per foot. See ADU kitchen and bathroom requirements.
• Solar. A newly built detached ADU generally needs a solar system under the state Energy Code, which can go on the ADU or the main house. Conversions and additions don't. See ADU solar and Title 24.
• Two stories or one. See two-story ADUs if the lot is small.

A pre-approved plan skips most of this. The City of San Diego gives applications that use pre-approved plans a 30-day review, though you still have to license the plan from its owner. In the unincorporated county, the County's free plans are about 85% complete and need site-specific details filled in. See pre-approved ADU plans in San Diego.

The permit

In the City of San Diego, ADU applications go in online through the Development Services application portal, under “Building Construction.” Your designer or builder normally submits and answers the plan-check corrections. State law puts the city on a clock:

Those deadlines come from the state's ADU handbook, which also notes that you can ask to pause the 60 days, for example while you revise plans. Coastal permits run on their own 60-day clock alongside the building permit. The full walkthrough, including what to submit, is in how to get an ADU permit in San Diego.

Construction and inspections

Once the permit is issued, the build is mostly your contractor's job. Yours is to approve changes in writing, pay only for work that's done, and keep an eye on inspections. City inspectors check the work in sequence: underground, foundation, framing, roof, rough plumbing and electrical, insulation, drywall and final. Nothing can be covered up before it's inspected.

Payments should track that progress. CSLB's ADU fact sheet gives a sample schedule:

The same fact sheet tells you to confirm who is handling the underground utilities, since trenching is easy to leave out of a bid, and to get every change order in writing. For the phase-by-phase detail, read the ADU construction process. If you're converting an existing outbuilding, shed to ADU conversions covers what has to be upgraded.

Final inspection and moving in

The last inspection is the one that matters most. In San Diego, final inspection is the approval to occupy, and it comes only after every other approval on the permit is signed off. Under state law, you need a certificate of occupancy before anyone lives in the unit. Hold back the final payment until you have it and the punch list is done.

Before a tenant moves in, know the local rental rules. The City of San Diego requires ADU rentals to be at least 31 days, so short-term vacation rentals are out. If you might sell or refinance later, keep the permit, the final inspection record and the contractor's lien releases together; ADUs and real estate explains why buyers and appraisers ask for them.

How long all of this takes varies with the city and the builder. Snap ADU estimates 10 to 18 months or more from first design to move-in in San Diego. The ADU timeline guide breaks that down by stage.

Common questions