Can I keep my existing garage slab for an ADU conversion?

Roughly 40% of the time, yes. The slab needs to be 4 in thick, within ±3/8 in of level, have functional reinforcement, and test under 4 lbs MVER on a moisture test. We do the assessment before any contract.

How much does it cost to prep a garage slab for conversion?

$1,500–$15,000 for a typical 400 sqft garage. Preserve + topical seal is $1.50–$3/sqft. Self-leveling pour-over is $4–$8/sqft. Full demo and re-pour is $18–$28/sqft.

Do I need a vapor retarder under the garage slab?

Yes — required by CBC §1907.6 for any living space. Existing garage slabs rarely have one, so we add either a sheet retarder under a self-leveling pour-over or a topical epoxy moisture-vapor coating.

What if my garage slab is sloped?

Garage slabs slope 1–2% toward the door for drainage. A self-leveling pour-over (3/4 to 2 in) corrects the slope and gives you a flat finished floor. Adds $4–$8/sqft.

Can I cut the existing slab for new plumbing?

Yes, with care. We mark existing rebar with ground-penetrating radar before cutting, trench only as needed, and patch with bonding agent and color-matched concrete after plumbing rough.

Will the new floor height match the rest of the house?

Usually not — the garage slab typically sits 4–8 in below the main house's floor. We address this with a transition threshold, a step, or a leveling pour-over depending on the layout.

How long does slab prep take?

1–2 weeks for assessment plus a self-leveling pour-over. 2–3 weeks for full demo, plumbing trenches, and a fresh slab pour with proper cure time.

Will my flooring warranty be honored over a converted garage slab?

Only if the slab is documented to meet the manufacturer's moisture and flatness specs. We provide the moisture test reading and flatness measurement in writing before the flooring installer starts.

Garage Slab Prep & Repair.

Make the existing slab livable — or know when to pour over.

An existing garage slab is the single biggest cost-saver in a garage-to-ADU conversion — but only if it can actually serve as a finished floor. We inspect every slab against four criteria (thickness, level, moisture barrier, structural integrity) and recommend preserve, patch-and-pour-over, or full demo. The decision shapes the entire project budget.

Typical range

$1,500 – $15,000 for typical 400 sqft garage

Per unit

$1.50 – $28 / sqft of slab

Timeline

1–2 weeks for assessment + pour-over; 2–3 weeks for full demo and re-pour.

The short version.

Most California garage slabs were poured between 1940 and 1985 to a 4 in nominal thickness with little to no vapor retarder and minimal rebar. The slab usually slopes 1–2% toward the garage door — fine for parking, problematic for finished flooring. The first step in any conversion is a slab assessment: thickness probe, level survey, moisture test, and crack mapping.

If the slab is 4 in thick, within ±3/8 in of level, has functional rebar, and tests under 4 lbs MVER on a moisture test, it's preserved as-is with a topical moisture seal. About 40% of garage slabs in our LA and Bay Area conversions fall into this category.

When the slab fails one or two criteria, the fix is a self-leveling pour-over (3/4 to 2 in of leveler) plus a sheet vapor retarder. About 45% of conversions use this approach. When the slab is too thin, too cracked, or sits over uncompacted soil, full demo and re-pour is required — adds $8K–$14K but eliminates risk.

What you can actually pick.

Preserve existing slab + topical seal
Pros — Cheapest option, no added floor height, fastest schedule.
Cons — Only works on slabs in good condition.
$1.50–$3 / sqftMatches existing slab life
Self-leveling pour-over (3/4–2 in)
Pros — Fixes most level and moisture issues, fast install.
Cons — Adds floor height, requires door jamb adjustment.
$4–$8 / sqft50+ years
Full demo + re-pour
Pros — Resets every variable, integrates new under-slab plumbing.
Cons — Most expensive, breaks the cost advantage of conversion.
$18–$28 / sqft50+ years

What we deliver.

Slab assessment — thickness core, level survey, moisture test, crack map
Existing-conditions report with photographs and recommendations
Crack repair with epoxy injection or polyurethane
Self-leveling pour-over with sheet vapor retarder if needed
Door-jamb and door-undercut adjustments for new floor height
New under-slab plumbing trenches cut into existing slab
Patch and seal trenches with bonding agent and color-matched concrete
Final moisture test and floor-prep sign-off before flooring install

The code parts most owners miss.

Living-space slabs require a vapor retarder per CBC §1907.6 — existing garage slabs almost never have one.
Slab must be within ±1/4 in over 10 ft for hardwood and LVP; ±3/8 in over 10 ft for tile.
Moisture vapor emission rate (MVER) must be < 3 lbs/1,000 sqft/24 hr for most flooring.
Trenching that exposes rebar at < 1 in cover triggers structural review.
Garage-floor drains (if present) must be capped and abandoned per plumbing code.

Why getting this right pays off.

The garage slab decision is where conversion projects either deliver the promised cost advantage or quietly turn into expensive remodels. A correct preserve-as-is call saves $6K–$12K versus pour-over. A wrong preserve-as-is call leads to flooring failure inside 18 months and a tear-out / re-pour that costs $25K and weeks of work.

The decision tree is not optional. We do the assessment on every garage conversion, document it in writing, and price the project against the actual slab condition.

What goes wrong — and how to avoid it.

Assuming the slab is fine because it looks fine — moisture issues are invisible until flooring fails
Pouring leveler without a vapor retarder — locks moisture under the floor
Forgetting to drop the door jamb for the new floor height — door won't close
Cutting trenches through unmarked rebar — weakens slab, expensive structural fix
Skipping the moisture test before flooring — voids manufacturer warranty

After we hand you the keys.

Re-seal topical moisture coating every 7–10 years if exposed
Monitor any new cracks for movement over time
Keep landscape grading sloped away from the converted garage perimeter
Inspect door-undercut clearances seasonally for swelling or settling

In short.

Can I keep my existing garage slab for an ADU conversion?: Roughly 40% of the time, yes. The slab needs to be 4 in thick, within ±3/8 in of level, have functional reinforcement, and test under 4 lbs MVER on a moisture test. We do the assessment before any contract.
How much does it cost to prep a garage slab for conversion?: $1,500–$15,000 for a typical 400 sqft garage. Preserve + topical seal is $1.50–$3/sqft. Self-leveling pour-over is $4–$8/sqft. Full demo and re-pour is $18–$28/sqft.
Do I need a vapor retarder under the garage slab?: Yes — required by CBC §1907.6 for any living space. Existing garage slabs rarely have one, so we add either a sheet retarder under a self-leveling pour-over or a topical epoxy moisture-vapor coating.
What if my garage slab is sloped?: Garage slabs slope 1–2% toward the door for drainage. A self-leveling pour-over (3/4 to 2 in) corrects the slope and gives you a flat finished floor. Adds $4–$8/sqft.
Can I cut the existing slab for new plumbing?: Yes, with care. We mark existing rebar with ground-penetrating radar before cutting, trench only as needed, and patch with bonding agent and color-matched concrete after plumbing rough.
Will the new floor height match the rest of the house?: Usually not — the garage slab typically sits 4–8 in below the main house's floor. We address this with a transition threshold, a step, or a leveling pour-over depending on the layout.
How long does slab prep take?: 1–2 weeks for assessment plus a self-leveling pour-over. 2–3 weeks for full demo, plumbing trenches, and a fresh slab pour with proper cure time.
Will my flooring warranty be honored over a converted garage slab?: Only if the slab is documented to meet the manufacturer's moisture and flatness specs. We provide the moisture test reading and flatness measurement in writing before the flooring installer starts.

Keep reading.

Fire Separation Wall (Garage to Living)
1-hour fire wall between the new ADU and the existing house.

Planning garage slab prep & repair?

Send us the address and the scope. We'll come back with a line-item budget, a permit path, and a realistic schedule — before you spend on drawings.

Start a project →