What is a typical net-to-gross ratio for a housing site?

It varies widely by site. Small infill plots can stay 80 to 90 percent developable, medium sites often land around 60 to 80 percent, and large strategic sites carrying spine roads, open space and drainage can fall to 50 to 60 percent or lower. Always derive it from the specific site constraints rather than a single rule of thumb.

Is density measured on gross or net area?

Density (dwellings per hectare) is conventionally quoted as net density, on the net developable area. Applying a net density figure to the gross site area is a common mistake that overstates unit count, GDV and land value.

What reduces net developable area the most?

On larger sites the biggest consumers are usually roads, public open space, and drainage or SuDS, followed by landscape buffers and biodiversity net gain land. Site-specific constraints such as flood zones, easements and protected trees can remove further area.

Why does developable area matter before GDV?

Because GDV is unit count multiplied by value, and unit count is net developable area multiplied by density. An error in developable area multiplies straight through into GDV and land value, so it has to be resolved before any residual calculation.

Gross to net developable area: the number behind every land bid

Threshold Guides · 9 min read · England & Wales

Every site has two areas that matter, and confusing them is one of the quickest ways to overpay for land. Gross site area is everything inside the red line. Net developable area is the smaller part you can actually build homes on. This guide explains what eats the difference between them, the ratios to expect, and why you have to resolve developable area before any GDV or land-value maths.

Gross area versus net developable area

Gross site area is the full extent of the site as drawn. Net developable area is the land given over to the developable use: the housing plots and their private curtilage, and in some definitions the immediately associated access. Everything that supports the scheme but is not saleable plot comes out. This is why experienced buyers price and compare land per net developable acre or hectare, or per plot, rather than per gross acre. Gross area flatters the unit count, and a headline price per gross acre can hide a poor deal.

What eats the difference

On most sites the gap between gross and net is taken up by a familiar set of demands:

Roads and access. The internal spine and estate roads, junctions, visibility splays and footways.
Public open space and amenity. Policy commonly requires a proportion of the site as open space, play areas and informal green space.
Drainage and SuDS. Sustainable drainage now expects attenuation basins, ponds, swales and easements, which can take a meaningful slice of a site, especially on flat or clay ground.
Landscape and buffers. Structural planting, boundary screening, and buffers to watercourses, woodland or neighbouring uses.
Biodiversity net gain. Since it became mandatory, most schemes must demonstrate a measurable net gain in habitat, and on-site habitat land competes directly with plots.
Constraints and easements. Flood zones, gas and drainage easements, overhead line corridors, tree protection and root protection areas, public rights of way, steep gradients, ransom strips and contamination hotspots.
Utilities infrastructure. Substations, pumping stations, and bin and cycle stores.

Typical net-to-gross ratios

The ratio is site-specific, but ranges help you sense-check a plan:

Small infill sites often keep a high proportion developable, sometimes 80 to 90 percent or more, because they need little internal infrastructure.
Medium sites commonly land in a broad middle band, often around 60 to 80 percent.
Large and strategic sites, carrying spine roads, significant open space, drainage and green infrastructure, can fall to 50 to 60 percent or lower.

Treat any single rule of thumb with suspicion. Two five-hectare sites can have very different net areas depending on flood risk, topography, ecology and local open-space policy.

Take a five-hectare (about 12.4-acre) greenfield site. At a 55 percent net-to-gross ratio the net developable area is 2.75 hectares. At a net density of 35 dwellings per hectare, that is roughly 96 homes. Had you instead applied 35 dwellings per hectare to the full gross area, you would have forecast about 175 homes, nearly double. Every downstream number, the unit count, the GDV and the land bid, would have been overstated by that same margin.

Density: net versus gross

Density (dwellings per hectare, or dph) is conventionally quoted as net density, measured on the net developable area, not the gross site. Mixing the two is a common and expensive error. As a rough guide, lower-density suburban housing often sits around 25 to 40 dph, higher-density and urban housing around 40 to 75 dph, and town-centre apartment schemes can exceed 75 and run well beyond 100 dph. Your achievable density is set by local character, planning policy, parking standards and the mix you are proposing. Density and mix also interact with value: more units of a denser product can beat fewer large units on revenue per acre, but only within what the location and policy will accept.

Why this is the foundation of the appraisal

GDV is unit count times value, and unit count is net developable area times density. Get the developable area wrong and the entire residual land value is wrong, because the error multiplies through the unit count into GDV and straight into land value. A twenty percent overstatement of developable area is a twenty percent overstatement of your maximum land bid, which is more than enough to lose money on a site. This is the single most important reason to resolve developable area before you touch the value maths.

Getting developable area right in practice

Work from constraints, not aspirations. Map the flood zones, easements, trees, access points and topography first, because they are fixed. Benchmark the layout against built-out local schemes of similar size and policy context, whose achieved net densities are real evidence rather than hope. Keep a contingency, because early layouts almost always lose plots as drainage, open space and biodiversity requirements firm up. Then feed the resulting unit count and floor areas, not the gross-area optimism, into your value work and your unit-mix decisions. Threshold prices the homes for you, turning 4.6 million sold records into achievable rates per square foot, comparables and a maximum land bid; you supply the developable area and mix that convert those rates into a real scheme.

Frequently asked questions

What is a typical net-to-gross ratio for a housing site?: It varies widely by site. Small infill plots can stay 80 to 90 percent developable, medium sites often land around 60 to 80 percent, and large strategic sites carrying spine roads, open space and drainage can fall to 50 to 60 percent or lower. Always derive it from the specific site constraints rather than a single rule of thumb.
Is density measured on gross or net area?: Density (dwellings per hectare) is conventionally quoted as net density, on the net developable area. Applying a net density figure to the gross site area is a common mistake that overstates unit count, GDV and land value.
What reduces net developable area the most?: On larger sites the biggest consumers are usually roads, public open space, and drainage or SuDS, followed by landscape buffers and biodiversity net gain land. Site-specific constraints such as flood zones, easements and protected trees can remove further area.
Why does developable area matter before GDV?: Because GDV is unit count multiplied by value, and unit count is net developable area multiplied by density. An error in developable area multiplies straight through into GDV and land value, so it has to be resolved before any residual calculation.

Run this on a real site in under 60 seconds

Threshold turns 4.6 million official sold records into comparables, £/sqft ground-pricing maps, price prediction and a maximum land bid, from one pin. Free 7-day trial, no card.

Start free trial →

Data referenced in Threshold is displayed under the Open Government Licence (HM Land Registry Price Paid Data, EPC Register, ONS, UK HPI). All figures produced by Threshold are statistical estimates for site appraisal, not RICS valuations.