Garage Conversion to ADU: Real Costs, by Region
A 2-car garage conversion runs $80,000-$140,000 turnkey in most US metros. A comparable detached ADU runs $230,000-$330,000. The gap is real. You’re not building a shell, a roof, or a slab. Those already exist. You’re paying for interior systems and finishes, not for creating a building.
The catch is that “comparable” is doing some work in that sentence. Your existing garage is probably not the size you’d choose if you were starting fresh. And the conversion cost assumes a garage in decent shape: sound slab, adequate ceiling height, electrical headroom. Where any of those assumptions break, costs climb toward detached-build territory.
Here’s what a garage conversion actually costs, what drives that number, and how to tell whether it’s the right call for your lot.
The line-item breakdown: 400 sq ft, 2-car garage, turnkey
The table below covers a standard 2-car attached garage (approximately 20x20 feet, 400 square feet) converted to a full ADU with a kitchen, bathroom, and sleeping area. Mid-range finishes. National anchor pricing; see the regional section for California.
| Phase | Line item | Cost |
|---|---|---|
| Pre-construction | Architectural plans | $5,000-$9,000 |
| Permit fees (building + plan check) | $4,000-$10,000 | |
| Structural engineer slab review | $1,500-$3,500 | |
| Site work | Utility trenching (if needed) | $3,000-$12,000 |
| Slab repair or reinforcement (if needed) | $0-$8,000 | |
| Shell | Garage door removal + exterior wall infill | $3,000-$6,000 |
| Framing (interior walls + ceiling) | $6,000-$14,000 | |
| Insulation (walls, ceiling, slab edge) | $4,000-$9,000 | |
| Windows + egress door | $3,000-$7,000 | |
| MEP | Plumbing rough-in + fixtures | $8,000-$18,000 |
| Electrical upgrade + rough-in + fixtures | $5,000-$12,000 | |
| HVAC (mini-split, single zone) | $5,000-$9,000 | |
| Finishes | Drywall + paint | $4,000-$8,000 |
| Flooring (LVP or tile) | $3,000-$8,000 | |
| Kitchen (cabinets + counters + appliances) | $12,000-$28,000 | |
| Bathroom (tile, vanity, fixtures) | $10,000-$22,000 | |
| Interior doors + trim | $2,000-$5,000 | |
| GC overhead + margin (~20%) | $12,000-$28,000 | |
| Contingency (10%) | $6,000-$14,000 | |
| Total | $97,000-$210,000 |
The low end assumes clean conditions: sound slab, no trenching required, an electrical panel with room for a sub-panel, and modest finishes. The high end reflects difficult site conditions, a full panel upgrade, and mid-high kitchen and bath specs. (Source: GatherADU 2026 garage conversion cost guide, gatheradu.com; Maxable garage conversion cost breakdown, maxablespace.com.)
The Maxable numbers are worth looking at directly: they put the starting point for a basic Southern California garage ADU (design, permits, and construction included) at around $140,000, with real-world projects ranging from $160,000 to $240,000 depending on finishes and site conditions. (Source: Maxable, maxablespace.com/what-does-a-garage-conversion-cost.)
A 400-square-foot detached ADU in the same markets runs $180,000-$280,000 nationally and $230,000-$330,000 in California’s coastal markets. For how those costs break down by size and finish level, the national ADU cost guide has the full picture.
Why the conversion costs less
Three things drive the cost gap, and they also explain when the gap shrinks.
The shell already exists. The foundation is poured, the walls are framed, the roof is on. Those are the most expensive parts of any building. On a garage conversion you skip all of it. No concrete pours, no shell framing, no roofing. That saves $60,000-$120,000 compared to a new build of the same footprint.
No new sewer lateral, usually. Running a new lateral from a detached ADU to the city main costs $8,000-$20,000 depending on distance and lot conditions. A garage conversion typically ties into the existing home plumbing over a shorter run on known soil.
No excavation. A new detached build starts with grading, excavation, and foundation prep. On sloped lots that’s significant money. The conversion skips it entirely.
The savings are real and consistent. What eats into them is the gap between what your existing garage provides and what a habitable ADU requires.
The four places conversions go over budget
1. Foundation and slab problems. Garages were built to a looser standard than habitable space. Older garages often have 4-inch slabs poured without a vapor barrier, no perimeter insulation, and footings sized for a lightweight structure. If your slab is cracked, settled, or structurally inadequate, a structural engineer will flag it. Repairs add $8,000-$25,000 before any interior work starts. This is the most common conversion surprise and the hardest to catch from a surface inspection. Get a structural engineer to look at the slab before you budget. The inspection runs $500-$1,500 and is the best money you’ll spend before committing to a scope.
2. Ceiling height. Under IRC R305.1, habitable space requires a minimum ceiling height of 7 feet. (Source: 2021 International Residential Code, Section R305.1.) That 7 feet is measured from the finished floor to the finished ceiling: after insulation, drywall, ducts, and any beams. A garage with 8 feet of raw clearance typically finishes at around 7’4”-7’6”, which clears the requirement. A garage with 7 feet of raw clearance will land at roughly 6’4”-6’8” finished. That fails the habitable-space standard, and raising the roof costs more than building new. Measure the raw ceiling before spending money on design drawings.
3. Parking replacement. Before you convert, verify what your city requires in place of the lost garage parking. In California, cities cannot require replacement parking when an existing garage is converted to an ADU. They also cannot require any ADU parking at all when the property is within 1/2 mile walking distance of a public transit stop. (Source: California Government Code §65852.2.) Outside California, the rules vary by city. Some jurisdictions require off-street replacement parking elsewhere on the lot. If your lot has no room for it, the conversion may be approvable only as a non-rental space, or not at all. Check your city’s ADU ordinance directly. Don’t rely on a contractor’s summary of it.
4. Expanding the footprint. A conversion that stays inside the existing walls is simpler and usually faster to permit. Push a wall out or add floor area, and you’re doing an addition. That means a foundation extension, additional framing, more complex structural review, and in many cities, a slower permit track. If your existing garage is 260 square feet and you need 400 square feet to make the unit functional, the addition cost and timeline start converging with a small detached ADU. The cost math changes considerably at that point.
Regional cost variation
The national anchor holds across most of the Midwest and South. Three regions move the number significantly.
California. The state’s 2025 energy code (Title 24) applies to garage conversions that change a space to habitable use. That typically requires upgrading insulation to current standards and adding mechanical ventilation. Solar panels are required for new ADU construction but may or may not be required for a conversion, depending on the scope of work and local amendments. Verify with your city’s building department before assuming either way. Bay Area labor adds 25-40% above the national baseline.
GatherADU’s 2026 regional breakdown for a full 2-car garage ADU conversion: Los Angeles $95,000-$175,000; San Francisco $120,000-$200,000; San Diego $85,000-$150,000; Sacramento $70,000-$125,000. (Source: GatherADU regional cost breakdown, gatheradu.com.) The California ADU cost guide has the full breakdown with permit fee specifics and Title 24 line items.
Northeast. Boston and New York metro areas push conversion costs 20-35% above the national anchor. Older housing stock tends to mean genuinely inadequate garage electrical: outdated wiring, undersized panels. That adds $10,000-$20,000 to the electrical scope before the interior work has started.
Midwest and South. Most markets in the Midwest and non-coastal South fall within the national anchor range. Permit fees vary 4x between cities in these regions, which makes a quick call to the building department worth your time before you finalize a budget.
[IMAGE NEEDED: Side-by-side floor plan comparison of a 400 sq ft garage converted to an ADU versus a 400 sq ft detached new-build ADU on the same lot, with cost callouts on each showing where the money goes]
When the conversion is right, and when the detached build wins
The conversion wins when the garage gives you enough space for the actual use case, is in sound condition, and you don’t need it for parking. For a single-person rental, a home office, or a studio for a family member, 400 square feet often works at roughly half the cost of a detached ADU.
The detached build wins when you need 600-plus square feet, the structure has condition issues, or the unit needs a layout and orientation the garage can’t provide. The ADU vs. garage conversion guide walks through the full decision framework. For the attached-versus-detached design question more broadly, this piece on detached vs. attached ADUs covers what drives that choice before you pick a contractor.
Financing the conversion
The smaller loan amounts make financing a conversion simpler than financing a new build in most cases.
HELOC. Most homeowners who’ve held their home for five or more years have enough equity to cover an $80,000-$140,000 conversion with a home equity line of credit. As of early 2026, HELOC rates for strong-credit borrowers run 7-9% APR. You draw as you build and pay interest only on what’s been disbursed. This is the most common path.
Cash-out refinance. Works if your existing mortgage rate is in the 6%+ range. If you’re sitting below 5%, raising your rate on the full balance to fund a conversion rarely pencils out. Run the numbers before assuming it’s worth it.
Personal or credit union loan. For conversions under $80,000, a personal loan at 8-11% APR is worth modeling. No equity required, faster to close than a HELOC. The tradeoff is a shorter repayment term (typically 5-7 years) and higher monthly payments than a HELOC.
Construction loan. Less common for conversions than for new builds, but available from some lenders. Draws follow the inspection schedule, so you pay interest only on what’s been disbursed. Most lenders require the completed value to support the full loan amount, which is easier to underwrite when the conversion produces a permitted rental unit with documented income potential.
Three things to do before you call a contractor
Measure the ceiling. Stand in the garage and measure from the concrete floor to the underside of the roof structure. You need at least 8 feet of raw clearance to reliably clear the 7-foot habitable-space minimum under IRC R305.1 after insulation and drywall. At 7’6” raw it may still be achievable with a careful assembly. Verify that with a designer or structural engineer before committing to drawings.
Check your city’s parking rules. Search “[your city] ADU parking replacement requirements” and read the ordinance, not a contractor summary. California homeowners near transit are generally covered by Gov Code §65852.2. Everywhere else, call the building department and ask directly.
Get a structural engineer to assess the slab. Not a general contractor. A licensed structural engineer. They’ll tell you whether the slab and footings need work and what it’ll cost before you’ve spent money on plans that may need revision. Budget $500-$1,500 for the assessment.
Those three steps will tell you whether the conversion is feasible, how much the hidden-cost items add to your budget, and whether your number is realistic for your market. After that, get two contractor estimates on equivalent scopes and compare the line items. The totals can look close even when the scopes are not.