Parts for this step · tap one to find it
A detached two-car garage, about 20 × 20 ft on its existing slab, turned into a studio ADU with a kitchenette and bathroom. The white parts are the existing garage. Every new part is below, in the order the steps use it. Nails and small hardware are not listed.
Build time depends on the plans, inspections and trades
This guide shows the sequence, not a design. Sizes, spacing, connections and energy values come from your approved plans and local code. Check them before cutting anything.
A conversion starts on paper. The plan set shows the layout, the new openings, the plumbing, electrical and mechanical work, and the Title 24 energy report, which sets the insulation, window and HVAC requirements. The building department approves it before work starts.
Design and plan check often take weeks to months · Plan set + Title 24 energy report · permit before work starts
Parts: 1 set × Plans, permit and Title 24 energy report
California law generally lets an existing garage become an ADU where it stands, without new setbacks or replacement parking. Cities must act on a complete ADU application within 60 days. Utility connections, fire separation and design rules still vary by city.
Garage slabs were poured for cars, not living space. Many have no vapor retarder underneath, and most slope toward the door so water drains out. Before the floor plan is final, the slab is tested for moisture and checked for cracks and level.
≈1–3 days, including probe settling time · 3 test probes · ≈370 sq ft of floor inside the walls
Parts: 3 × Slab moisture (RH) test probe
The right moisture protection depends on the slab and the plans. This example uses a vapor-retarding coating and leveling just before flooring (step 9). Other plans call for a raised wood floor over a vapor barrier. Flooring makers set their own moisture limits. (The model hides the roof while we look inside.)
The garage door, tracks and opener come out. A new wall fills the 16 ft opening under the existing door header: a pressure-treated bottom plate anchored to the slab, studs 16 in on center, and a framed window opening.
≈1 day · Studs 16 in on center · 16 ft opening · window rough opening 60 × 36 in
Parts: 1 × Pressure-treated bottom plate, 2×4 × 16 ft; 4 × Concrete anchor (size and type per plans); 16 × Stud, 2×4 × 8 ft; 1 × Top plate, 2×4 × 16 ft; 1 × Window header, doubled 2×6 (from one 12 ft board)
Wood that sits on concrete must be pressure-treated. Many cities also want a short concrete curb or a new footing under this wall to keep driveway water off the wood; the plans say which. The existing header over the old door still carries the roof, and this wall fills in below it.
The entry door and the sleeping area’s egress window go into the side wall. That wall holds up the roof, so the roof is shored temporarily while studs come out and headers go in. Header sizes come from the plans; a doubled 2×8 is this example.
≈1 day · Door opening 38½ × 82½ in · window opening 36 × 48 in
Parts: 11 × Stud, 2×4 × 8 ft; 2 × Door/window header, doubled 2×8 (one 8 ft board each)
An egress window needs at least 5.7 sq ft of net clear opening (5.0 sq ft at a ground-floor window), at least 24 in clear height and 20 in clear width, and a sill no more than 44 in above the floor. The window maker lists the net clear opening. Here the sill is 34½ in above the slab.
The window and door units go in with flashing. The infill wall gets a weather barrier and new stucco that ties into the old finish. Around the side-wall openings, the cut-back stucco is patched.
≈3–5 days, with curing time between stucco coats · 3 openings · ≈115 sq ft of stucco patch
Parts: 3 × Sill pan and flashing, per opening; 1 × Window, 5 × 3 ft slider; 1 × Egress window, 3 × 4 ft casement; 1 × Exterior door, 36 in prehung; ≈110 ft² × Weather barrier (building paper); ≈115 ft² × Stucco patch: lath and three coats
Flashing and fastening follow the window and door makers’ instructions. Where the plans call for sheathing on the new wall, it goes on before the paper. An exact color match on old stucco is hard, so plan to repaint at least the whole wall.
Plumbing, electrical and heating/cooling go in while the walls are open. Licensed trades do this work, and each part is inspected before anything is covered. This guide shows where it goes, not how.
≈1–2 weeks across the trades · Plumbing · electrical · mini-split heat pump · each inspected
Parts: 1 × Plumbing trench in the slab; 1 set × Drain lines (licensed plumber); 6 × Electrical box; 1 set × Wiring (licensed electrician); 1 × Subpanel, where the plans call for one; 1 × Mini-split line set
The ADU may need an upgraded electrical service. For a conversion inside an existing structure, state law generally bars the city or water district from requiring separate water or sewer connections, though an owner can choose them. The model leaves out vents, water lines and the bathroom walls so you can see in.
Insulation fills every exterior wall cavity and the ceiling, to the R-values in the Title 24 energy report. It’s inspected before any drywall goes up.
≈1 day · ≈510 sq ft of wall · ≈370 sq ft of ceiling
Parts: ≈560 ft² × Wall insulation, R-15 batts for 2×4 walls; ≈405 ft² × Ceiling insulation, R-value per energy report
Common prescriptive values for a small conversion are R-15 in 2×4 walls and R-30 or R-38 in the ceiling, depending on climate zone. Your energy report sets the real numbers. Some crews drywall the ceiling first and blow insulation in from above. The model cuts the ceiling away at the back so you can see the walls.
Drywall goes on the ceiling first, then the walls, and the joints are taped and finished. The bathroom gets moisture-resistant board, and the shower gets a tile backer.
≈3–5 days with taping · 33 sheets, 4 × 8 ft (≈18 walls + ≈12 ceiling, plus 10%)
Parts: 33 × Drywall, 4 × 8 ft sheet
Ceiling joists 24 in apart usually take 5/8 in drywall or ½ in sag-resistant ceiling board. Some cities inspect drywall fastening before taping.
The slab gets its moisture protection and flooring. Then the cabinets, fixtures, mini-split and alarms go in, and each trade finishes its work. The city’s final inspection comes before anyone moves in.
≈1–3 weeks · ≈370 sq ft of floor · flooring includes about 10% waste
Parts: ≈370 ft² × Vapor-retarding slab coating; ≈405 ft² × Flooring (vinyl plank in this example); 1 × Kitchenette cabinets and sink; 1 × Shower base and glass; 1 × Toilet; 1 × Vanity and sink; 1 × Mini-split indoor unit; 1 × Mini-split outdoor unit; 1 × Smoke alarm
Coverage and moisture limits come from the coating and flooring makers. Some cities also assign the ADU a separate address.
That’s a garage turned into a studio ADU: the same slab, walls and roof, now insulated, weather-tight and permitted for living in.