How we estimate ADU costs & feasibility
Every number on Grannio is a transparent planning estimate — never a quote. Here is exactly how we calculate it, so you can judge how much weight to give it.
Construction cost
We start from national turnkey cost ranges per square foot for each ADU type (including design, permits and site work), drawn from 2024–2025 ADU cost surveys, then scale them by a regional cost index for your state (national average = 1.00) that reflects local labor and materials pricing.
| ADU type | National $/sq ft (turnkey) |
|---|---|
| Detached new build | $220–$360 |
| Attached addition | $200–$330 |
| Garage conversion | $120–$220 |
| Junior ADU (JADU) | $100–$180 |
| Prefab / modular | $180–$300 |
The mid-point is split into ~70% construction, ~18% design & permits, and a type-dependent site-work share (lowest for conversions and JADUs, highest for detached new builds).
On city pages we apply an additional metro adjustment for higher- and lower-cost markets (for example, San Francisco runs above the California average), so city estimates are more local than the state baseline.
Feasibility & rules
12states (plus a growing list of others) have a statewide ADU law that limits what cities can restrict. For those, we apply the published statute's standards (size guarantees, setbacks, parking and owner-occupancy) and cite the law on each page. For states without one, we say plainly that local zoning governs — we never invent a statute. We also flag lot-coverage risk when a unit's footprint is large relative to the lot you enter.
Rental income & ROI
Estimated rent uses a national ADU rent band per square foot scaled by the same regional index, with a damped size adjustment (small units rent at a higher $/sq ft). Gross yield is annual rent ÷ build cost; simple payback is build cost ÷ annual rent after a 7% vacancy allowance, before financing and maintenance. The financing figure is a standard amortized monthly payment on the build cost over 20 years at an illustrative 7.5% rate — adjust for your actual loan terms.
Important
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.