Most ADU calculators ask for a square footage and multiply it by a statewide number. That tells you nothing about your lot. Parcy is an ADU calculator that starts from the parcel: it pulls your lot lines and zoning, places a unit in the buildable area, and prices that unit with data from your neighborhood.
What it calculates
| Output | How Parcy gets it |
|---|---|
| Build cost | Local per-square-foot pricing by ADU type (detached, attached, converted, JADU), stories, and finish level, plus a soft-cost allowance. |
| Rent | Live rental comps within a short radius of the lot, matched to the unit's bedrooms and size. |
| Value added | Sales comps for 1 to 3 total units, a 60/40 income-and-comps blend at 4 units, and the income approach (5% vacancy, 35% expenses, market cap rate) at 5 or more. |
| Return on cost | Value added divided by build cost. The number that decides whether to proceed. |
| Unit count | State ADU law for single-family lots and SB 1211 for multifamily, applied to the actual lot geometry. |
Why area data matters
An 800 square foot ADU in Torrance and the same unit in Fresno differ by $100,000 in cost and $1,500 a month in rent. A calculator that cannot tell them apart will tell one owner to build when they should not and the other to wait when they should not. Parcy's numbers move when you move the pin.
ROI versus return on cost
Return on cost is value created per dollar spent, before financing. ROI in the app layers in rent: at a 40% return on cost and a 7% rental yield on cost, a leveraged ADU on a multifamily lot often returns the owner's cash in three to five years through a refinance. The app shows both so you can underwrite the way a lender will.
Try it on your address
Parcy is free on iPhone and Android. Enter the address, drag the unit where you want it, and watch the numbers update. Export a PDF when you want to share it with a partner or lender.
Frequently asked questions
Is there a free ADU calculator for California?
Yes. Parcy is a free app that calculates ADU build cost, rent, value added, and return on cost from your actual parcel and local data.
How do you calculate ADU ROI?
Value added divided by total build cost gives return on cost. Add annual net rent divided by cost for cash yield. Parcy computes both from local comps and pricing.
Informational only. Not legal, architectural, or investment advice. Confirm current rules with your local planning department.