Subdivision lot yield calculator

How the lot count is worked out

1. Start with the gross acres and take out the land you can't build on: floodplain and wetlands, stream and perimeter buffers, easements, and slopes too steep to build on. What's left is the net buildable area.
2. Set aside the open space the zoning requires, as a share of the gross acres. Some ordinances let floodplain and buffers count toward it. Tick the box if yours does.
3. Take out streets and right-of-way and stormwater ponds, each as a share of the land that's left. The rest is land in lots.
4. Divide the land in lots by the average lot size. That's the first limit.
5. Check the street frontage. The street area divided by the right-of-way width gives the length of street. With homes on both sides, the frontage is twice that, and dividing it by the minimum lot width gives the second limit.
6. If the zoning caps homes per acre, that's the third limit. The lowest of the three, rounded down, is your yield.

A worked example: 40 acres

Take a 40-acre tract with 4.5 acres of floodplain and wetlands, 2 acres of buffers, 1.5 acres of easement and an acre too steep to build on. That's 9 acres out, leaving 31 buildable.

The zoning wants 10% open space, so 4 acres come out, leaving 27. Streets take 20% of that, 5.4 acres, and stormwater ponds 6%, another 1.62. That leaves 19.98 acres in lots, about 870,300 square feet.

At an average lot of 11,500 square feet, that's 75 lots. The 5.4 acres of street at a 50-foot right-of-way is about 4,700 feet of street, enough frontage for 134 lots at 70 feet wide, so lot area is the limit. The yield is 75 lots, 1.88 homes per gross acre and 2.42 per buildable acre.

Half the site ends up outside the lots. And every acre of floodplain or buffer that wasn't in the numbers costs about 2.8 lots.

Which limit sets your count

The calculator works out three counts and keeps the lowest, and the result tells you which one it was. Knowing that tells you what to change.

Lot area is the usual limit. Smaller lots, or less land in streets and ponds, add lots. Street frontage takes over when lots are wide for their size: in the example, a 120-foot minimum width drops the frontage limit from 134 lots to 78, close to the 75 that fit by area. Narrower lots, or a layout with more street, add lots then. A density cap takes over when the zoning allows fewer homes than the land holds. At a cap of 1.5 homes per gross acre the same 40 acres yield 60 lots, and only a rezoning or a density bonus adds more.

The open space rule matters more than it looks. If the ordinance lets the floodplain, wetlands and buffers count toward the 10% open space, the example's 9 constrained acres cover it, and the yield rises from 75 lots to 86.

How much land the streets take

With homes on both sides, each lot needs half its width in street length, so the street area a lot carries is half its width times the right-of-way width. A 70-foot lot on a 50-foot right-of-way carries 35 × 50 = 1,750 square feet of street, about 13% of the lot and its street together for an 11,500-square-foot lot.

That's the floor. Entrance roads with no homes on them, intersections, cul-de-sac bulbs and streets with homes on one side all add to it, which is why the calculator starts at 20% of the land left after open space. The lots per acre guide has the street share for common lot widths.

State rules that cut yield

Stream buffers are the state rule that most often costs lots, and the minimums differ by state. Counties and cities often require wider ones, so check the local ordinance too.

A buffer along a stream that winds through the middle of a tract can cost far more lots than the same acreage along an edge, so map the streams before you trust any count. In North Carolina's water supply watersheds, the density cap can set the yield before lot size does: put it in the density cap field.

Terms used here

Gross acres

The whole tract, as deeded. Land is priced and listed on gross acres.

Net buildable acres

What's left after floodplain, wetlands, buffers, easements and steep slopes come out.

Open space

Land the zoning requires you to leave out of lots, as a share of the site. Some codes let constrained land count toward it.

Right-of-way

The strip a street sits in: the pavement plus the curb, sidewalk and utility strips on each side.

Frontage

The length of a lot's line along the street. The minimum lot width sets how much each lot needs.

Density cap

The most homes per acre a zoning district allows, on gross acres or net acres depending on the code.

Gross density

Homes divided by the whole tract.

Net density

Homes divided by the buildable acres. Some codes divide by the land in lots instead.

Yield

How many lots a site holds once every rule and constraint is applied.

Assumptions

• The acres you can't build on don't overlap. If a buffer sits inside the floodplain, count that land once.
• Streets and ponds are shares of the land left after open space. The 20% and 6% here are placeholders, and your engineer's layout will differ.
• The frontage check assumes homes on both sides of every street. Entrance roads and single-loaded streets have less frontage than that.
• The density cap applies to gross acres. If your ordinance counts net acres, adjust the cap to match.
• The example numbers are round figures to show the math. They don't describe any real site.

For lots per acre at common lot sizes and everything that cuts a yield, read how many lots per acre a subdivision holds. To turn the lots into a land price, use the residual land value calculator, and to see how REOS measures a site's constraints, see site analysis.

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