Accessory dwelling units — backyard cottages, garage apartments, basement suites — are one of the fastest ways to add housing on land that is already developed. For builders, lenders, insurers and investors, that makes them a market worth tracking. Doing it with data takes all three layers: permits, zoning and parcels.
The trap: “ADU” is not one thing
Search Loudoun County’s permits for “ADU” and most of what comes back is not a backyard cottage. In Loudoun, ADU usually means Affordable Dwelling Unit — income-restricted homes built inside new subdivisions under the county’s affordable-housing program. Permit descriptions in the Buildstats catalog show both kinds side by side:
- a three-level ADU townhouse in a new planned community — affordable;
- finishing a room above a barn as a self-contained unit for employee housing — accessory;
- new 200-amp electrical service to an accessory dwelling — accessory.
Count keyword hits without reading them and you will overstate the accessory market badly. The fix is to read the permit text — every Buildstats record keeps the full work_description — and to check the ordinance itself.
Step 1: Know where ADUs are allowed
Whether an accessory dwelling is allowed by right, needs a special permit, or is prohibited depends on the zoning district — and each ordinance defines the use in its own words. ZoneOptics reads Loudoun’s ordinance into districts and permitted uses, with the ordinance’s own definition of each use, so you can tell a by-right lot from a special-exception one before anyone files a permit.
Step 2: Find the lots that qualify
Zoning tells you the rule; parcels tell you which lots meet it. Land Records carries the zoning code, land use, acreage and existing building data for every parcel, so you can list single-family lots in ADU-eligible districts that are large enough to hold a second unit.
Step 3: Watch who builds
Permits close the loop. A Buildstats monthly subscription delivers every permit from the past 12 months, refreshed monthly — new accessory dwellings, the electrical and mechanical permits that follow them, and the contractors doing the work.
Who uses this
- Builders and prefab ADU makers: eligible lots without an ADU are the pipeline.
- Lenders: renovation and construction loans for second units.
- Insurers: a second dwelling changes occupancy, value and risk.
- Planners and advocates: measure whether a zoning reform is actually producing units.
Get the data
Loudoun County is available now — full history or a monthly subscription. Browse every county we cover, or ask us for the one you need next.