How buildable acres are measured

Buildable acres is the land left on a parcel after taking out the parts you usually can’t build on. It’s the number that turns “40 acres” into “40 acres, of which 22 you can use”.

What it takes out

Four things, each from a public map:

Flood zone. FEMA’s 100-year flood zone (the 1% annual chance zone) and the regulatory floodway. The 500-year zone is shown on the map but not taken out, because building there is usually allowed.
Wetland. Forested, shrub, emergent and estuarine wetland from the US Fish and Wildlife Service’s National Wetlands Inventory.
Open water. Ponds, lakes, reservoirs and rivers from the USGS National Hydrography Dataset.
Steep ground. Ground steeper than 15%, worked out from USGS 3D Elevation Program data. 15% is roughly where grading costs start to climb and many ordinances start to restrict building.

A spot that is two of these at once (a wetland in the flood zone, say) is taken out once, not twice. So the parts on the parcel card always add up to the whole parcel.

How it’s measured

Each county is measured in one pass:

The flood zones, wetlands, water and elevation for the whole county are laid onto a grid of cells about 10 metres across.
Each cell is marked with what it is: flood, wetland, water, steep, or none of these.
Each parcel outline is laid onto the same grid, and its cells are counted.
Buildable acres is the parcel’s acres times the share of its cells that are none of the four.

Road frontage is measured in the same pass: the parcel’s boundary is walked in 10-metre steps and every step within about 15 metres of a mapped road centerline (30 metres for US and NC routes, whose right of way is wider) counts as frontage.

Where it can be wrong

It’s a screening number, built for comparing thousands of parcels quickly. Know its limits:

Small parcels. The grid is about 10 metres, so on a parcel under 2 acres a single cell moves the percentages a few points. Parcels under 2 acres aren’t measured at all.
Map age. The wetland inventory in parts of North Carolina dates from the 1980s and 1990s. New ponds, filled wetlands and recent flood map changes may not be there.
What the maps don’t show. Stream buffers, septic suitability, easements, rock and ordinance setbacks aren’t taken out. Some of them are map layers you can turn on.
Sliver flood zones. A thin strip of flood zone along a creek can be a few cells wide or none, depending on where the grid falls.

For a purchase, get a survey and, where wetland is likely, a wetland delineation.

Words used on this page

Buildable acres: The parcel's mapped acres minus the parts in the FEMA 100-year flood zone or floodway, mapped wetland, open water, and ground steeper than 15%. Each spot is only taken out once, even where two of these overlap.
Road frontage: The length of the parcel's boundary that runs along a public road centerline, in feet. Zero means no mapped road touches the parcel: it may be landlocked or reached by an easement.