ADU
Garage Conversion to ADU — Complete Cost Guide 2026
Garage-to-ADU conversion is the most cost-effective path to add a legal rental unit in 2026 — typically 30–50% cheaper than building a detached ADU from scratch. But the "save by converting" pitch hides a lot of compliance work: foundation reinforcement, fire-rating walls if attached, parking waivers, separate utilities, and ceiling-height retrofits. Here's the honest 2026 breakdown.
Garage conversion cost by garage size (2026)
- 1-car garage (240–300 sqft): $85,000 – $140,000 total
- 2-car garage (440–520 sqft): $120,000 – $210,000 total
- 3-car garage (640–720 sqft): $165,000 – $290,000 total
Per sqft, that's $280–$480/sqft installed. Cheaper than detached ADU construction by $40–$140/sqft, but expensive relative to the assumption that "the walls and roof are already there."
Why is conversion cost so high? Where the money goes
For a 480 sqft 2-car garage conversion at a national-average $165,000 total:
- Foundation work — $4,000 – $12,000. Most pre-1990 garages have 4" thin-slab unreinforced concrete designed for car loads, not residential dead + live loads. Either reinforced with rebar-pinning + topping slab, or replaced entirely (more common). Insulated under-slab.
- Insulation upgrade — $4,800 – $8,400. Garages are uninsulated. New conversion needs R-13 walls, R-30 ceiling/attic, R-10 under-slab. Closed-cell spray foam in walls is typical for the air-sealing benefit.
- Drywall + finishes — $7,200 – $11,800. Full drywall, taping, mudding, sanding, two coats paint. Garage interiors are typically primer-only or exposed framing.
- Floor finish — $3,200 – $7,800. Garage slab is sloped 1/8" per foot to a drain. ADU floor needs level. Either self-leveling underlayment ($2.50–$4/sqft) or raised subfloor system (TG plywood on sleepers).
- Ceiling-height retrofit — $0 – $14,000. Most garages have 8' or 8'4" ceilings. ADU code requires 7'6" minimum in habitable rooms — fine. But if you're converting a low-ceiling garage (some older homes have 7'2"), you need to raise the roof which is expensive.
- New windows + sliding door — $4,800 – $9,200. Egress window required in any sleeping room (5.7 sqft minimum clear opening, 24" minimum height); typically a full window + slider replacement of the garage door opening.
- HVAC — $7,400 – $13,200. Either a ductless mini-split system (most common in conversions — no return-air duct complexity) or a small heat pump tied to the main home's system with new ducts.
- Electrical — $9,600 – $16,800. Typically a new 100-amp sub-panel from main house, GFCI outlets throughout, dedicated kitchen + bath circuits, AFCI breakers, LED-spec lighting plan.
- Plumbing rough-in + fixtures — $12,800 – $22,400. The single biggest line item. Includes: extending main water + sewer to the garage location, all interior supply + DWV, washer/dryer hookup, full bath fixtures, kitchenette plumbing. Add $4,800–$9,600 if water heater + sewer line require longer trenches.
- Kitchen + bath cabinetry + counters — $11,400 – $19,800.
- Appliances — $4,800 – $8,400. Apartment-grade fridge, range, dishwasher, washer/dryer (stacked).
- Fire-rating (if attached to main home) — $3,200 – $7,400. Wall between ADU and main home requires 1-hour fire-rated assembly (5/8" Type X drywall on both sides + fire-blocking + self-closing fire door).
- Architectural + permits — $6,400 – $14,800. Stamped plans, permit fees, impact fees, plan check, inspections.
- GC markup + project management (18-22%) — $24K–$36K.
Cost-saving plays that actually work
- Keep the garage door opening visible (slider behind it). Saves $4,800 in stucco/siding + structural header replacement. Common code-compliant approach in CA, OR, WA.
- Mini-split HVAC instead of central air. Saves $4,800–$7,200 in ducting + plenum + AHU costs. Visually it's a wall cassette — modern designs are sleek and ADU buyers don't object.
- Tie utility connections to main home, not new utility-company meters. Saves $9,000–$15,000 in separate-meter fees + trenching. Most jurisdictions allow this for owner-occupied + ADU configurations.
- Stack washer/dryer in a closet, not a dedicated laundry room. Saves 14–22 sqft of habitable space + plumbing simplification.
- Use prefab kitchenette ($4,800–$7,200) instead of full custom ($11K+). Pre-fab ADU kitchenettes from companies like Cottage, Abodu, and Cover come with cabinets + sink + cooktop + fridge + dishwasher in one delivered unit. Quality has improved dramatically in 2024-2026.
The 4 states where garage ADUs are by-right (auto-approved)
- California (AB 68, AB 671 + AB 2533): Any single-family lot. Cities CANNOT require parking replacement, owner-occupancy, or design review for garage conversions specifically.
- Oregon (HB 2001 + HB 2003): By-right in any single-family zone in cities >2,500 population.
- Washington (HB 1110, effective 2024): Up to 2 ADUs by-right on any residential lot in cities >25,000 pop. Garage conversions explicitly protected.
- New Hampshire (RSA 674:73): By-right in 56 of 234 municipalities — check your specific town.
In these four states, the permit timeline drops from 10–14 months to 2–4 months — saves $8,000–$18,000 in carrying costs vs traditional discretionary review.
The detached vs converted-attached decision
If your garage is detached (separate building in backyard), conversion + adding a sleeping room + bath retains the privacy + soundproofing of detached ADU AND comes in 30–40% cheaper than ground-up detached construction.
If your garage is attached to the main home, you save on shared wall + foundation + utilities — but lose the privacy + soundproofing of detached. Rental rates for attached-converted units run 10–18% below detached cottages per Zumper 2026 data. Net IRR is usually slightly better for detached because of higher rental rates outweighing higher construction cost.
Permit & approval timeline (2026)
- By-right states (CA, OR, WA, NH select towns): 60 days from complete application
- States with discretionary review: 4–9 months
- HOA-controlled communities: Add 30–90 days for HOA approval (and ~20% denial rate)
Bottom line
Converting a 2-car garage into a legal ADU costs $120K–$210K in 2026 — substantially more than the "we already have walls + roof" narrative suggests, because foundation + insulation + plumbing + permit work are essentially the same as ground-up. The conversion still saves 25–40% vs detached new construction and dramatically accelerates timeline in by-right states. Best ROI for empty-nesters, multi-gen households, or any owner-occupant in a high-rent metro.
Compare detached ADU: ADU Construction Cost by State 2026
Sources & methodology
Cost data from 2026 NAHB Cost Estimating Survey ADU module, Houzz ADU Pro Pricing 2026, ADU specialists in CA (Abodu, Cottage), OR (Habitat for Humanity ADU+), and WA. State-by-state legal review from California HCD, Oregon DLCD, Washington Commerce, and NAR State Housing Policy Database 2026.