ADU Cost in California (2026): Garage Conversions and Detached Units

The short answer

In Southern California, a garage-conversion ADU typically costs $120,000–$220,000 and a detached ADU of about 500 square feet $180,000–$300,000. Detached units of 800–1,000 square feet typically run $280,000–$450,000. A useful rule of thumb for detached units is roughly $300–$450 per square foot all-in, with conversions lower.

National ADU averages sit well below California numbers, and the Bay Area generally runs about 10–20% above Southern California for the same unit.

Cost by ADU type

What California law says about ADU permits

California has made ADUs much easier to permit. Under Government Code §66317, an ADU application must be reviewed ministerially, without a discretionary hearing. The agency has 15 business days to tell you whether the application is complete, and it must approve or deny a complete application within 60 days when there is an existing home on the lot. If it does not act in time, the application is deemed approved.

State law also limits what cities can require. According to California HCD guidance, a city cannot require replacement parking when a garage, carport or covered parking structure is converted into an ADU, cannot set a maximum ADU size below 850 square feet (1,000 for more than one bedroom), and ADUs up to 750 square feet are exempt from impact fees. Local details still matter, so every project starts with a feasibility check.

What drives ADU cost

• Conversion vs. new build. Reusing a sound garage saves foundation and framing, but slab, footing and wall condition decide whether it can be reused.
• Utilities. Trenching distance to the main house, sewer connections and an electrical panel upgrade can add tens of thousands.
• Site. Slopes, poor soil and fire-hazard zones add engineering and materials.
• Size and plumbing. Each bathroom and kitchen adds fixtures, venting and inspections.
• Finish level. Flooring, cabinetry and fixtures follow the same tiers as a main-house remodel.

Timeline

The whole journey typically takes about 6–12 months from first meeting to move-in. Design and engineering come first, then plan check (where the 60-day clock applies once the application is complete), then construction and utility connections. Utility company scheduling is a frequent wildcard.

Garage conversion or detached ADU?

If you are deciding between an ADU and expanding your home, read about room additions, which typically cost about $300–$500 per square foot for ground-floor work.

Budgeting tips

• Get a site and utility assessment before design to avoid surprises.
• Keep plumbing walls stacked and the footprint simple.
• Choose durable, low-maintenance finishes for rental units, such as luxury vinyl plank or microcement floors.
• Ask whether your size qualifies for impact-fee exemptions.
• Include a contingency for utility work.

A note on how we work: MDG Builders sells, designs and manages each project, and the hands-on work is performed by licensed, insured subcontractors in each trade. You get one accountable point of contact, one schedule and one written proposal.

Learn more on our ADU construction page or request an ADU consultation.

Line items in an ADU budget

An all-in ADU budget includes far more than construction. Expect design and architectural plans, structural engineering, a survey where needed, energy calculations, city plan-check and permit fees, utility work, construction, finishes and appliances, and a contingency. Utility costs vary the most: a short sewer run and an adequate electrical panel can keep them modest, while a long trench, a sewer lateral replacement or a service upgrade can add significantly.

Ask every builder whether their number includes design, permits and utilities. Construction-only numbers look attractive but are not comparable to an all-in figure.

Choosing the right size

State law prevents cities from capping ADU size below 850 square feet, or 1,000 square feet with more than one bedroom, per HCD guidance, and ADUs up to 750 square feet are exempt from impact fees. That 750-square-foot line is worth noting when you plan: staying at or under it can avoid impact fees entirely, while larger units pay fees proportional to the main home.

Bigger is not always better for rental returns. A well-designed one-bedroom of 500 to 750 square feet often rents efficiently, while each additional bathroom adds meaningful cost. Design efficiency, such as stacked plumbing, simple rooflines and standard window sizes, keeps the cost per square foot under control.

Junior ADUs

A junior accessory dwelling unit (JADU) is created within the walls of an existing single-family home, such as a converted bedroom suite, and is limited in size. JADUs typically cost far less than detached units because they reuse existing structure and utilities. California law allows JADUs alongside ADUs on many single-family lots, so some owners combine a garage conversion with a JADU. Local rules on owner occupancy and kitchens for JADUs apply.

Finishes that work for rental units

If you plan to rent, choose finishes for durability and easy turnover: luxury vinyl plank or sealed microcement floors, quartz counters, solid-surface or large-format tile showers, and mid-range fixtures with available replacement parts. Laundry in the unit and good storage are features renters value. Our flooring and kitchen pages explain options in more detail.

Financing an ADU

Homeowners commonly finance ADUs with cash, a home equity line of credit, a cash-out refinance or a renovation loan that considers the finished value. Some lenders consider projected rental income. Compare total borrowing costs, not just rates, and build your draw schedule around construction milestones. Program availability changes, so check with lenders and your local housing agency for current options.

Step-by-step: from idea to move-in

1. Feasibility: confirm zoning, setbacks, utilities and site constraints. 2. Design: floor plan, elevations and selections. 3. Engineering and energy calculations. 4. Plan check: submit, respond to comments, receive permits; the 60-day state clock runs from a complete application. 5. Construction: foundation, framing, rough-in trades, inspections, insulation, drywall, finishes. 6. Utilities and final inspection. 7. Certificate of occupancy and move-in. Clear decisions at each step keep the schedule moving.

Common ADU mistakes

The most expensive ADU mistakes happen before construction. Designing before checking utilities, setbacks and easements can force a redesign. Underestimating utility work is the most common budget miss. Choosing custom sizes for windows and doors adds cost and lead time. Skipping a contingency leaves no room for soil or sewer surprises. Finally, ignoring how the unit will be used, whether for family, rental or a home office, leads to layouts that do not fit. Spending time on feasibility and a clear brief is the cheapest insurance you can buy.

How to check a contractor in California

Before hiring anyone for a home improvement project in California, look up their license on the Contractors State License Board website. Confirm that the license is active, that the classification fits the work, and that bonding and workers' compensation information is on file. California generally requires a contractor's license once labor and materials on a job exceed a small dollar threshold set by the CSLB, and specialty work such as roofing is performed under its own classification, for example C-39 for roofing.

Ask for a written contract that describes the scope, materials, schedule, payment schedule and how changes are handled. California limits down payments on home improvement contracts, so be cautious of anyone asking for a large deposit up front. Read recent reviews on independent platforms and ask how communication works during the project: who your point of contact is, how often you will get updates, and how quickly questions are answered.

At MDG Builders, My Design Guide Inc holds CSLB license #1040337, Class B (General Building). We sell, design and manage each project, and the work is performed by licensed, insured subcontractors whom we schedule and supervise.

Is an ADU worth it?

For many California homeowners an ADU provides rental income, space for family or a home office, and added property value. The economics depend on your local rents, construction cost and financing. Run the numbers with a realistic all-in budget, including design, permits, utilities and finishes, rather than a construction-only figure.

Frequently asked questions

Related service: ADU Construction by MDG Builders.