Every ADU builder knows the drive out to a lot that was never going to work: the backyard is a slope, the side yard is four feet wide, or the parcel is in a zone that doesn't allow a detached unit. A short desk check before the visit saves that trip and lets you show up to the good ones with real answers.
This is the check we run on our own leads, in the order we run it.
1. Pull the parcel
Start with the county's parcel viewer or GIS. Get the parcel number, the lot lines and the lot size. Look at the aerial: where the house sits, where the open space is, and whether there are other structures (sheds, pools, a detached garage) taking room.
2. Confirm the zone and the ADU rules
Find the zoning district and read the local ADU section of the code, not just the summary page. Note:
- whether a detached ADU is allowed in that zone;
- side and rear setbacks for ADUs, and any separation from the main house;
- the height limit and any size limits;
- lot coverage or floor-area limits that count the existing house.
State law in California, Washington and Oregon sets floors for what cities must allow, but each city writes and applies its own code. Read the local one.
3. Draw the buildable area
Offset the lot lines by the setbacks and take out the existing buildings plus any required separation. What's left is the buildable area. Try your standard plans in it. If none fits, you know before the visit, and you can lead with an attached unit or a conversion instead.
4. Check access and slope
From the aerial and street view, look for a clear path to the back for equipment and materials. Check the terrain: a steep rear yard changes foundation cost and, often, sewer routing.
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5. Find the utilities
Locate the sewer main or septic, the water meter and the power service. Sewer is the one that most often moves the building, because the line needs fall. If the lot is on septic, the health department's rules come into play early.
What to send the homeowner
After the desk check, send a short note: what you found, which plans look like they fit, and what still needs a site visit to confirm. Homeowners respond to specifics about their own lot far more than to a brochure, and the ones who reply are the ones worth the drive.
Let the homeowner do the first pass
If your website lets a homeowner type their address and see their lot, the setbacks and which of your plans fit, most of this desk check arrives with the inquiry. That's why we built ADU Insights for our own company. Whatever tool you use, keep it honest: it's a first look, and the city still has the final word.