Units per hectare: how density is measured and used

A units per hectare density policy figure is an output of a design, not an input to it. Local plans still carry density ranges, and officers still test schemes against them, but the number only means something once you have said which hectares you divided by. Gross site area and net developable area can differ by half on a site with a watercourse, a spine road and a buffer to ancient woodland, which is enough to move a scheme from policy-compliant to under-developed on paper alone.

So state the arithmetic. Every density dispute I have seen started with two parties using different denominators and neither saying so.

Gross against net site area

Gross site area is everything inside the red line. Net developable area is the part that can carry housing and its immediate curtilage.

The usual exclusions from net area are:

• Land in functional floodplain, Flood Zone 3b, that cannot be developed
• Strategic open space, formal play provision and sports pitches required by policy
• Primary access roads serving more than the residential parcels, and any land for a spine road or junction improvement
• Significant landscape buffers, including stand-off distances to ancient woodland and railway or pipeline easements
• Land for a school, local centre or other non-residential use
• Existing woodland, hedgerows and watercourse corridors retained for ecological reasons
• Attenuation basins sized for the wider catchment

Incidental open space within a parcel, private gardens, parking courts and estate roads normally stay inside the net area. There is no single statutory definition, which is precisely why the schedule of areas matters more than the resulting figure.

Habitable rooms per hectare appears in some older plans and in London policy history. It rewards family housing less visibly than dwellings per hectare does, so check which measure the adopted policy actually uses before calculating anything.

How density ranges appear in local plans

Most adopted plans do one of three things. They set a minimum density for allocated sites or for defined accessibility zones, they give an indicative range by settlement type, or they say nothing numeric and rely on character and design policies instead.

Read the supporting text as well as the policy. A range expressed as an expectation, with the plan saying departures will be considered where justified by site constraints, is a different test from a minimum expressed as a requirement. Site allocations often carry their own indicative capacity, and where that figure came from a call-for-sites submission rather than a capacity study it may not survive contact with the constraints.

Why a density figure is an outcome not a target

The London Plan 2021 moved away from a density matrix to a design-led approach under its site capacity policy, and that thinking has spread well beyond London. The sequence is: establish the constraints, test built form options against local character and the standards that apply, then count the units that result.

Run it the other way and the scheme fails somewhere else. Push the unit count up and you lose private amenity space, daylight to the lower flats, or the parking the highway authority will insist on. The capacity falls out of the design once the standards are applied honestly, which is the argument a design and access statement has to make explicit.

Nationally, the NPPF (August 2026 edition) expects efficient use of land, supports minimum density standards where accessibility is good, and allows councils to refuse schemes that make insufficient use of land where there is a shortage of housing land. Cite the edition when you quote it, because the Framework was re-issued in December 2024 and again in August 2026.

Design led capacity against numeric density

In practice the two approaches meet in the middle. Officers want a number to compare against the plan. You want the number to be defensible.

The way through is a capacity statement: the net area calculation, the built form tested, the standards applied, and the resulting density stated against the policy range with the difference explained. Where the figure sits below the range, give the constraint that caused it. Where it sits above, show that daylight, amenity space and refuse storage still work.

An illustrative case: a 1.8 hectare site with a 0.25 hectare attenuation basin, a 0.2 hectare buffer to a stream corridor and 0.15 hectares of policy-required open space has a net area of about 1.2 hectares. Forty-eight dwellings is 27 per hectare gross and 40 per hectare net. Those two numbers describe the same scheme. The figures here are illustrative.

Density and accessibility to public transport

The policy logic links density to accessibility, because parking demand and trip generation are what constrain dense development away from good public transport. Where a site sits on a frequent bus corridor or near a station, expect the plan to look for more units and the highway authority to accept lower parking provision.

That link cuts both ways at scale. Once a residential scheme passes 50 units or 2,500m² it will usually need a transport assessment rather than a statement, and at higher densities the assessment has to deal with servicing, refuse vehicle tracking and cycle parking volumes that lower-density layouts never reach.

Density also drives the obligations. Affordable housing thresholds start at 10 or more units in national policy terms, and the tenure split and transfer arrangements go into an affordable housing statement. Push the unit count up and the contribution rises with it.

Where a low density scheme is refused

Refusals on under-development are real and increasingly common on allocated sites. The pattern: a site allocated for 60 homes comes forward for 35 large detached units, the officer calculates the density against the allocation, and the report concludes the proposal makes inefficient use of a limited supply of housing land.

Defending that position needs evidence rather than preference. Topography, a conservation area setting, tree constraints, flood risk, access limitations or an adopted character appraisal can all justify a lower figure. Market preference cannot.

There is a phasing trap too. Splitting a site to keep each parcel below a threshold, then building out the whole allocation at low density, invites refusal on both grounds, as we set out in phasing a large scheme without gaming thresholds.

Presenting the calculation

Put a schedule of areas on a drawing, not in the text. Colour the net developable area, list each exclusion with its area in hectares to two decimal places, and total it. Then give both gross and net density and say which the policy uses.

Three habits save arguments later. Use the same red line area as the application form. Keep the schedule consistent with the drainage and open space plans, because officers do cross-check. And update it when the layout changes, since the density quoted in a planning statement that no longer matches the submitted plan is the kind of discrepancy that delays validation.

Enter your postcode on PlanScope to see which reports the scale of your scheme is likely to trigger.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.