How buildable acres are measured

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

What it takes out

• Flood zone: FEMA’s 100-year flood zone and floodway. The 500-year zone is shown on the map but not taken out, since building there is usually allowed.
• Wetland: from the US Fish and Wildlife Service’s wetland maps.
• Open water: ponds, lakes and rivers, from USGS maps.
• Steep ground: slopes over 15%, from USGS elevation data. That’s roughly where grading gets expensive and many rules start to limit building.

A spot that is two of these at once is only taken out once, so the parts always add up to the whole parcel.

How it’s measured

Each county is laid onto a grid of squares about 10 metres (33 feet) across. Each square is marked flood, wetland, water, steep or none. Buildable acres is the parcel’s acres times the share of its squares marked none.

Road frontage is measured at the same time: the length of the parcel’s edge that runs along a mapped road.

Where it can be wrong

It’s a screening number, built for comparing many parcels quickly.

• Small parcels: one square moves the numbers on a small lot, so parcels under 2 acres aren’t measured.
• Old maps: some wetland maps are decades old. New ponds, filled wetlands or recent flood map changes may be missing.
• What the maps don’t show: stream buffers, septic suitability, easements, rock and setbacks aren’t taken out. Some are map layers you can turn on.

Before you buy, get a survey and, where wetland is likely, a wetland delineation.

Words used on this page

Buildable acres

The parcel's acres minus flood zone, wetland, open water and ground steeper than 15%.

Road frontage

How many feet of the parcel's edge run along a public road. Zero means it may be landlocked or reached by an easement.