Guide · garage conversions
A garage conversion is usually the cheapest legal route to an ADU in California, and not because it is smaller. State law removes, one by one, the requirements that make new construction expensive: the existing walls keep their setbacks (Gov. Code § 66314(a)(7)), the parking you lose never has to be replaced (§ 66314(d)(11)), fire sprinklers are not required unless the main house has them (§ 66323(d)), and under 750 square feet the unit pays no impact fees at all (§ 66311.5(c)(1)).
Most detached garages were built long before today's setback rules and sit closer to the property line than any new structure could. For a conversion, that does not matter:
“No setback shall be required for an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure that is converted to an accessory dwelling unit.”
Gov. Code § 66314(a)(7) · verified against leginfo
Note the second clause: you can even demolish and rebuild in the same location and to the same dimensions and keep the protection. A garage on the lot line stays on the lot line.
Parking is where conversions win twice. First, the spaces the garage provided are simply gone, with no obligation to recreate them anywhere: when a garage, carport, or covered parking structure is demolished or converted to build an ADU, the local agency may not require the lost off-street spaces to be replaced (Gov. Code § 66314(d)(11)).
Second, the ADU itself needs no parking either. Gov. Code § 66322(a)(3) bars any parking standard “where the accessory dwelling unit is part of the proposed or existing primary residence or an accessory structure”, and a garage is an accessory structure. Read § 66322 →
“The installation of fire sprinklers shall not be required in an accessory dwelling unit or a junior accessory dwelling unit if sprinklers are not required for the primary residence.”
Gov. Code § 66323(d) · verified against leginfo
If your house predates sprinkler mandates, the conversion cannot be made to trigger them. Sprinkler systems are one of the line items that most often surprises owners converting detached structures, and this is the section to cite when one appears in a correction list.
Nearly every garage conversion comes in under 750 square feet of interior livable space (a two-car garage is roughly 400 square feet), which puts it under the statutory impact fee exemption in Gov. Code § 66311.5(c)(1). School fees, park fees, traffic fees: none of them may be imposed. You still pay for plan check, the building permit itself, and utility work, which are service charges rather than impact fees. The distinction is covered in detail in the fees guide.
The slab and the walls. Garage foundations were poured for cars, not bedrooms. Insulation, moisture barriers, egress windows, and sometimes foundation work are where conversion budgets actually go, and no statute caps construction cost. Get the structure assessed before you fall in love with a floor plan; homeowners who did it in the other order describe paying for design twice.
Utilities. Trenching for water and sewer, and panel capacity for an all-electric unit, vary lot by lot and city by city. For junior ADUs, § 66338 bars treating the unit as a separate dwelling for connection fees; for full ADUs the connection fee rules are narrower, so get the utility's demand in writing early.
An unpermitted garage. If the garage itself was built or extended without permits, the city still cannot use that to refuse the ADU application unless it is an actual health and safety threat (§ 66322(b)). If it was built before 2020, the legalization path is stronger still.
The rules above are the statewide floor. What you can build also depends on your jurisdiction's compliance status and your lot. The feasibility check resolves both and returns a size range with the governing section for every figure.