Can you build an ADU? Find out in 60 seconds.
Check the accessory dwelling unit rules in your state — size, setbacks, parking and owner-occupancy — and get an instant cost estimate for a detached ADU, garage conversion or junior ADU.
Your property
ADU typeEstimated cost in California
$192,500 – $315,000
≈ $275–$450/sq ft turnkey · midpoint $254,000
- $172,700
- Construction
- $45,700
- Design & permits
- $35,600
- Site work
- ✓Size within the guaranteed minimum. Your 700 sq ft ADU is at or under the 800 sq ft your city must allow regardless of other rules.
- ✓4 ft setback. State law caps the side and rear setback at 4 ft, so even a tight lot usually works.
- ✓No extra parking likely required. No parking required if within 1/2 mile of public transit, in a historic district, or part of the existing/proposed primary residence.
- ✓No owner-occupancy requirement. You can rent out both the main home and the ADU — you do not have to live on the property.
- ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.
- ✓Ministerial approval in 60 days. By law your city must approve a complete ADU application within 60 days, without discretionary review or a public hearing.
- ✓Impact fees waived. ADUs under 750 sq ft are exempt from impact fees in this state — a saving of several thousand dollars.
Rule basis: Cal. Gov. Code §66314 et seq. (former §65852.2)
If you rent it out
$1,350–$2,175
est. rent/mo
8.4%
gross yield
12.8 yrs
payback
~$19,809/yr after a 7% vacancy allowance, before financing & maintenance.
Financed over 20 yrs at 7.5%, the build is about $2,046/mo — vs ~$1,775/mo in estimated rent.
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.
How Grannio works
Tell us your property
Pick your state, the ADU type you're considering and a rough size.
See feasibility + cost
We apply your state's ADU law and a regional cost index to flag what's allowed and what it'll cost.
Go further
Get a detailed feasibility report or connect with vetted ADU builders in your area.
ADU types & what they cost
Detached new build
$220–$360/sq ftA standalone backyard cottage built from the ground up. Most flexible and highest resale value, but the most expensive per square foot because everything — foundation, roof, utilities — is new.
Attached addition
$200–$330/sq ftAn ADU that shares one or more walls with the main house. Slightly cheaper than detached because it reuses an existing wall and is closer to utilities.
Garage conversion
$120–$220/sq ftConverting an existing garage into a living unit. The cheapest path because the foundation, walls and roof already exist — you mostly add insulation, plumbing, a kitchen and a bathroom.
Junior ADU (JADU)
$100–$180/sq ftA junior ADU is carved out of the existing home (up to 500 sq ft), often a converted bedroom with an efficiency kitchen. Lowest cost and fastest, but smaller and usually requires owner-occupancy.
Prefab / modular
$180–$300/sq ftA factory-built unit delivered and installed on a prepared site. Faster and more price-predictable than site-built; the range includes delivery, crane and site work.
Get your detailed feasibility report
A personalised PDF with your exact size and setback allowances, fee estimates, a phased budget and the documents your city requires — plus an intro to vetted ADU builders near you.
Financing your ADU
A handful of loan and grant programmes are built specifically for ADUs. These are informational links we don't currently earn a commission on — not paid placements.
RenoFi
Compare RenoFi ADU loan options →Renovation-specific loans (home equity loan, HELOC or cash-out refinance) that lend against your home's value after the ADU is built, so you can often borrow more than a standard HELOC allows.
Fannie Mae HomeStyle Renovation
See HomeStyle Renovation →A conventional mortgage that rolls ADU construction costs into a single loan, with down payments as low as 3% for qualifying first-time buyers.
CalHFA ADU Grant Program
Check CalHFA grant status →Reimburses up to $40,000 of pre-development costs (design, permits, soil tests) for California homeowners building an ADU.
Current round of funding is fully allocated — the page covers what to check for reopening.
Want financing options matched to your project?
Tell us roughly what you need to borrow and we'll point you to lenders who work with ADU projects.
ADU rules by state
✓ = statewide ADU law that limits local bans.
ADU guides
How Much Does an ADU Cost in 2026? Real Numbers by Type and State
What an ADU actually costs in 2026 — by construction type (detached, garage conversion, JADU, prefab) and by region, plus the hidden soft costs people forget.
Garage to ADU Conversion Cost: What You'll Actually Pay
Converting a garage is the cheapest way to add an ADU. Here's what a garage-to-ADU conversion really costs in 2026, what drives the price up, and when it beats a detached build.
California ADU Rules in 2026: Size, Setbacks & Parking
A plain-English guide to California's statewide ADU law in 2026 — the 800 sq ft guarantee, 4 ft setbacks, the transit parking exemption and the 60-day approval rule.
ADU vs JADU: Which Backyard Unit Should You Build?
ADU vs JADU explained — the size limits, cost difference, owner-occupancy rules and rental potential of a junior ADU versus a full accessory dwelling unit.
Frequently asked questions
How accurate is the ADU cost estimate?
The calculator uses 2024–2025 turnkey cost ranges per ADU type, scaled by a regional construction-cost index for your state. It returns a realistic low–high range for planning. Your final price depends on your specific lot, finishes and contractor — always get a local quote.
Can I build an ADU on my property?
In states with a statewide ADU law (like California, Washington and Oregon), almost every single-family lot qualifies for at least one ADU by right. Elsewhere it depends on local zoning. Our calculator flags the rules that apply to your state and the things to confirm with your city.
What's the difference between an ADU and a JADU?
A JADU (junior ADU) is up to 500 sq ft and must be built inside your existing home, usually with an owner-occupancy requirement. A full ADU can be detached, attached or a garage conversion, larger, and in most states rentable without living on-site.
Is Grannio free?
Yes — the feasibility checker and cost calculator are free. We offer an optional paid detailed feasibility report and can connect you with vetted ADU builders in your area.