A garage conversion ADU turns an existing garage into a permitted home with its own kitchen, bathroom, and entrance. Hamilton Exteriors ADU projects in the Bay Area start at $150,000 and can reach $600,000. A garage conversion usually sits at the lower end of that range, because the slab, walls, and roof already exist. This guide covers cost, California rules, the build process, and the cases where a garage conversion is the wrong choice.
Garage conversion ADU at a glance
- Cost: our ADU projects start at $150,000 and can reach $600,000. Garage conversions usually sit at the lower end.
- Typical size: 400 to 500 square feet from a two-car garage.
- Schedule: design and permitting take 2 to 4 months. Plan for 10 to 14 weeks from permit approval to move-in.
- Parking: California law does not let a city require replacement parking when a garage becomes an ADU.
- Setbacks: an existing garage converted in the same location and to the same dimensions needs no new setback.
How much does a garage conversion ADU cost in the Bay Area?
Our ADU projects start at $150,000 and can reach $600,000, including design, permits, and construction. A garage conversion usually costs less than a detached new-build ADU of the same size, because the foundation, walls, and roof are already there. A large detached unit with high-end finishes sits at the upper end of the range.
The final price depends on what the existing garage needs. These items move the number the most:
- Slab condition. Garage slabs often need moisture protection, leveling, or insulation before flooring.
- Utility distance. A detached garage far from the sewer line and electrical panel needs longer trenching and runs.
- Electrical capacity. An older panel can need an upgrade to carry a kitchen, heating, and laundry.
- Structure. Weak framing, a low ceiling, or the garage door opening can require engineering and new framing.
- Finish level. Cabinets, tile, windows, and fixtures scale with the finish you choose.
For prices by ADU type and by city, see our Bay Area ADU cost guide.
California rules that help garage conversions
State ADU law overrides many local limits. Four rules matter most for a garage conversion. Each one is in the California Government Code.
- Replacement parking is not required. When a garage is converted to an ADU, the local agency cannot require that the lost off-street parking spaces be replaced (Gov. Code section 66314(d)(11)).
- Existing setbacks stay. A city cannot require a setback for an existing structure that is converted to an ADU in the same location and to the same dimensions (section 66314(d)(7)).
- Fire sprinklers follow the main house. An ADU does not need fire sprinklers if the primary residence does not need them (section 66314(d)(12)).
- The city has 60 days to decide. The permitting agency must approve or deny a complete ADU application within 60 days when a home already exists on the lot (section 66317).
A conversion can also include an expansion of up to 150 square feet beyond the existing structure when the added space is needed for entry and exit (section 66323). Cities still apply building, energy, and fire codes, and each city has its own submittal process. We confirm the current local requirements before permit submission.
What the conversion involves
A garage was built to store cars. A home has to meet a different standard. A permitted conversion usually includes this work:
- Replace the garage door with a framed wall, windows, or an entry door.
- Protect the slab from moisture, then insulate and finish the floor.
- Insulate walls and ceiling to meet the energy code.
- Add fire separation where the garage is attached to the main house.
- Run plumbing for a kitchen and bathroom, and connect to the sewer.
- Upgrade electrical service and add permanent heating and ventilation.
- Add windows for light, air, and emergency escape.
The process, step by step
- Assess the existing garage. We measure the slab, ceiling height, framing, openings, access, and utility routes. This shows the repairs and design limits before drawings begin.
- Design and engineer. The plan sets the layout, windows, entry, kitchen, bathroom, and mechanical systems. Engineering and energy documents support the permit package.
- Permit. Design and permitting together usually take 2 to 4 months. Review time differs by city.
- Build. Construction often takes six to ten weeks once permits and materials are ready. Structural repairs, utility upgrades, and inspection timing can extend the schedule.
- Final inspection. The city signs off, and the unit is a legal dwelling.
When a garage conversion is the wrong choice
A conversion is not right for every property. Look hard at a detached ADU or a different plan in these cases:
- You need the garage. The law removes the parking requirement. It does not replace your storage, workshop, or covered parking.
- The structure is in poor condition. A cracked slab, failing framing, or a very low ceiling can push repair costs close to the cost of new construction.
- The garage is small. A one-car garage yields a compact studio. If you need a one-bedroom or two-bedroom unit, a detached ADU gives more design freedom.
- Utilities are far away. Long sewer and electrical runs to a detached garage at the back of a deep lot add real cost.
- You want more rental income. A larger detached unit usually rents for more than a converted garage.
A site review answers these questions early, before you pay for permit drawings.
Garage conversion, JADU, or detached ADU
| Type | Where it sits in the $150,000 to $600,000 range | Good fit |
|---|---|---|
| Junior ADU (inside the home) | Lower end | Smallest scope, space within the existing house |
| Garage conversion ADU | Lower end to middle | Sound existing garage, faster schedule |
| Detached new-build ADU | Middle to upper end | More space, full design freedom, higher rent |
Frequently asked questions
Do I have to replace the parking when I convert my garage?
California Government Code section 66314(d)(11) says a local agency cannot require replacement of off-street parking when a garage is converted to an ADU. We still confirm the current local review for your property before permit submission.
Can both attached and detached garages become ADUs?
Yes. An attached garage needs careful fire separation from the main home. A detached garage can need longer utility runs. We review the structure, access, and service connections before we recommend a plan.
How long does a garage conversion take?
Plan for 2 to 4 months of design and permitting. Construction often takes six to ten weeks once permits and materials are ready. Most projects reach move-in 10 to 14 weeks after permit approval.
Is a garage conversion cheaper than a detached ADU?
Usually, yes. Our ADU projects start at $150,000 and can reach $600,000. A garage conversion usually sits at the lower end because you reuse the existing foundation, walls, and roof. The tradeoff is less design freedom and a smaller unit, usually 400 to 500 square feet for a two-car garage.
Can part of the garage stay as parking or storage?
Sometimes. The remaining area must still work with fire separation, access, and utility requirements. We measure the garage and test partial-conversion layouts before we recommend this approach.
Next step
Hamilton Exteriors is an architect-led general contractor (CSLB #1078806). We handle design, engineering, permits, utilities, and construction for garage conversions across the Bay Area. See our garage conversion ADU service or call (650) 977-3351 to schedule a site review.
