Method

How this works, and where it stops

This page is the whole method, including the parts that make us look less impressive. If you are going to trust a number about your own property, you should know exactly where it came from.

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Your result first

Aerial: USGS/USDA NAIP

Step 1 — Your parcel boundary

You enter an address. We geocode it through Riverside County's own geocoder, then query the county assessor's public parcel layer for the polygon containing that point — the same data the county uses, not a commercial resale of it.

What can go wrong. The county's parcel layer strips street right-of-way, so a point that lands in the road returns nothing at all rather than an error. We have reproduced this on a real Murrieta address. When it happens we say we could not match your lot, rather than showing you a boundary that is not yours.

Step 2 — The buildable area

We shrink your parcel inward by the setback, then subtract two things:

The setback band. For a new detached ADU, California caps the side and rear setback at 4 feet. Your city cannot require more, so we use that figure.
Your existing house, plus separation. Buildings need clearance between them. We use 6 feet, which is a conservative assumption rather than a verified city value — it is the single hardest number to source per city and the one most likely to be wrong, so we err toward showing you less space than you may actually have.

What remains is the buildable area — the green shape in the drawing.

A real limitation, stated plainly. We shrink the parcel evenly on all sides rather than identifying which edge fronts the street. Front yards usually require a much deeper setback than 4 feet, so our buildable area is slightly too generous near the street. Doing frontage properly needs road centerline data, which is a later project. Until then we disallow front-yard placement in the tool, which removes the error rather than hiding it.

If we cannot find your house footprint — and outside Temecula we usually cannot, because the published building data in this county is poor — the buildable area only accounts for setbacks. That makes it too generous, and we tell you so on the result rather than letting you assume otherwise.

Step 3 — Fitting a plan

You pick a plan and we place its footprint at true scale. Then we search a grid of positions and two orientations across the buildable area for the best placement there is, and snap the plan to it.

That search matters more than it sounds. Without it the result would tell you whether you happened to drop the rectangle in a good spot, not whether the ADU fits at all. We caught this in our own testing — a 20 by 20 studio on a 6,600 sq ft lot reported "tight" purely because of where it had been dropped.

Step 4 — Why three answers instead of yes or no

3–25ftTypical gap between a public parcel line and the surveyed one
4ftThe setback we are measuring against it
4%Riverside County's stated acreage vs its own polygon, on one parcel

The error bar is wider than the thing being measured. That 4% is not a criticism of the county — assessor data exists for taxation, not for siting buildings — but given all three numbers, a confident yes would be a guess wearing a suit. So:

Result	What it means
Looks like it fits	The footprint clears the buildable area with at least 10 feet to spare — comfortably more than the data's error bar.
Tight	It fits on paper, but the margin is inside the measurement error. This is a genuine maybe. A surveyor or builder needs to confirm on site, and we will say so rather than pretending otherwise.
Unlikely as drawn	The footprint does not fit the buildable area. A smaller plan, a garage conversion, or an attached unit may still work.

We calibrated the 10 foot threshold empirically rather than picking a round number. At 3 and 5 feet almost everything came back as "fits", which is exactly the overconfidence the parcel accuracy data warns against. At 10 feet the results discriminate properly: large lots pass everything, typical tract lots pass smaller plans and flag large ones, and genuinely small lots honestly flag everything.

What this does not check

Any one of these can stop a project that passes the geometry check completely.

Utility and drainage easements, which often run through rear yards
Topography and slope
Sewer or septic capacity
Electrical panel capacity and gas availability
Soil conditions

This is not a survey and not a zoning determination. It is a planning estimate built from public records. Only a survey, a site visit, and your city's plan check produce an answer you can build on. What this tool is genuinely good for is deciding whether the idea is worth pursuing at all — which is a question most people currently answer by giving their phone number to a salesperson.

What happens after

Nothing, unless you ask.

The lot check is free. We do not need your name or phone number to show you the result.
Want the written report? We email it to you.
Want a builder? We introduce you to one licensed builder who works in your city, and we tell you who before we do it. Not five. Your phone will not start ringing.
Before any of that, you talk to a person here — partly so you get useful answers, partly self-interest: we would rather not send a builder someone whose project was never going to happen.

Read the FAQ

Now put it to the test

You have read the method and the error bars. Free, and no phone number.