My garage floor is lower than the house. Do I have to raise it for an ADU?

Not necessarily. You are generally not required to bring a converted garage floor up to your house's floor level, but the finished floor has to work as living space: usable and reasonably level, protected from slab moisture, and still leaving the minimum ceiling height the California Residential Code requires for habitable rooms. Whether you keep the slab, top it, or build a raised wood floor over it changes how plumbing, ducts, and wiring get routed.

Why garage floors are a problem

• Garage slabs are often set lower than the house floor and sloped toward the vehicle door so water drains out.
• Older slabs may not have a vapor retarder under them, so moisture can move up into new flooring and walls.
• The old door opening has to be closed with a new wall, which needs a proper base and a detail that keeps water out where the slab meets outside grade.

Your three basic options

1. Keep the slab and correct it. Leveling compounds or patching can fix modest slope. Building departments commonly ask how slab moisture is handled; Patterson's garage conversion handout, for example, requires concrete floors to be sealed with an approved vapor barrier sealer.
2. Pour a topping slab. A new slab over a vapor retarder corrects slope and raises the floor, but it adds weight and uses up ceiling height.
3. Build a raised wood floor on sleepers. This can bring the floor up to the house level and creates a cavity for some piping and wiring. Milpitas's building policy for garage conversions requires sleepers and sill plates in contact with concrete to be naturally durable or preservative-treated wood; where underfloor ventilation cannot be provided, it calls for a Class I vapor retarder over the slab with the voids insulated. It also says items that must stay accessible, such as plumbing cleanouts and electrical junction boxes, cannot sit in the reduced-clearance space.

Check ceiling height before you choose

Every inch you add to the floor comes out of headroom. Measure from the planned finished floor to the planned finished ceiling, including the new ceiling insulation and drywall and any soffits for ducts, and confirm the result still meets the code minimum for habitable rooms. A garage with a low ceiling or a dropped beam may have very little room to spare.

Mind the transitions

If an attached ADU's floor stays lower than your house and the two connect, the step at that door becomes a building-code detail for your designer, including landings and the fire-rated door between units. At exterior doors, the threshold and landing have to be detailed together with the floor height you choose.

What it means for your MEP plans

• Plumbing: Staying on the slab means saw-cutting and trenching for drains. A raised floor can sometimes carry short drain runs above the slab if there is enough depth for the required slope, with cleanouts kept accessible.
• Mechanical: Ductless heads avoid floor ducts entirely. A ducted system needs a ceiling or soffit route that still clears the minimum ceiling height.
• Electrical: Junction boxes must remain accessible, so wiring in a shallow sleeper floor is usually kept to the walls.
• Title 24: The energy consultant models the floor as it will actually be built, so settle the floor detail before the report is run.

Lock in the floor approach on the architectural plans first; the MEP routes follow from it. See the garage conversion guide, plumbing challenges in a garage conversion, and garage conversion MEP plans.

Sources: milpitas.gov, pattersonca.gov, hcd.ca.gov.