Land availability assessments and how sites are judged

A land availability assessment is the document that decides, long before any allocation appears in a draft plan, whether your site is treated as part of the supply. It is evidence rather than policy, which means nobody consults on it in the way they consult on a plan, and it is nonetheless the point at which most sites are quietly set aside. Reading your authority's land availability assessment planning evidence, and understanding the judgements inside it, is the most useful hour a promoter can spend at the start of a plan review.

What the assessment does

The assessment identifies a pool of sites and broad locations capable of contributing to the authority's housing and economic development needs, assesses each one, and produces an estimate of how much land is realistically available over the plan period.

It is a technical exercise with a published methodology, usually agreed through a duty to cooperate group so that neighbouring authorities use comparable assumptions. The output is a schedule, typically a spreadsheet with a row per site, carrying a reference, an area, an estimated capacity and a conclusion.

That conclusion is where the consequence sits. A site recorded as suitable, available and achievable enters the supply. A site recorded as having potential subject to constraints may be revisited. A site recorded as not suitable usually disappears from the process, and it takes a plan review or a persuasive application to bring it back.

The assessment also feeds the five-year housing land supply calculation, which is why its assumptions matter on applications as well as on allocations. Our note on housing land supply and what it changes for applicants covers the application side of that.

The methodology national guidance sets out

National guidance on housing and economic land availability assessment describes a staged approach and leaves the detail of each stage to the authority.

Two features of that structure are worth understanding. The guidance separates the question of whether a site could be developed from the question of whether it will be, which is why suitability, availability and achievability are assessed as distinct tests rather than rolled into a single judgement. And it expects the assessment to be reviewed and updated, so a conclusion reached four years ago is not fixed.

Authorities publish their own methodology document alongside the assessment. Read it before reading the site schedule, because it contains the density assumptions, the net developable area ratios, the constraint thresholds and the lead-in times that determine every entry.

Stages of site assessment

The staged sequence in the guidance runs roughly as follows.

1. Identify sites and broad locations, drawing on the call for sites, current and lapsed planning applications, the brownfield land register, site allocations in the existing plan, surplus public land and employment land reviews.
2. Assess each site for suitability, availability and achievability, and estimate its capacity.
3. Consider whether a windfall allowance for sites not individually identified can be justified.
4. Review whether the assessed supply meets the requirement, and if it does not, revisit broad locations, constrained sites and the possibility of a wider search.
5. Publish the evidence base, with the methodology and the assumptions stated.

Stage 4 is the one promoters should watch. Where the supply falls short, authorities go back to sites previously set aside, and a site with a resolved constraint is far better placed in that second pass than one whose file still says access unresolved.

Constraints that rule a site out

Some constraints are absolute in practice, some are a matter of degree, and the methodology document usually says which the authority treats as which.

Access and drainage are the two constraints a promoter can actually fix with evidence. Where a site is recorded as having no safe access and you hold the frontage, a visibility splay drawing resolves it. Where the issue is surface water, the authority's own mapping is the starting point, and our note on checking surface water and critical drainage areas locally explains how to read it.

Designations are different. Arguing about Green Belt or a National Landscape inside a land availability assessment is arguing about the plan strategy in the wrong document, and the right forum is the plan consultation.

Capacity assumptions and density

Capacity is where the arithmetic quietly decides outcomes, and it is the part of the assessment most open to challenge on evidence.

Three assumptions drive the number. The density applied, usually expressed in dwellings a hectare and often varied by location type. The ratio of net developable area to gross site area, which strips out roads, open space, drainage and constrained land. And any deduction for the constraints recorded against the site.

Authorities apply these as broad-brush rules across hundreds of sites, which is reasonable at that scale and frequently wrong on a specific site. A site with an existing frontage, no on-site open space requirement because of a nearby park, and surface water handled off-site can support a materially higher capacity than the standard ratio produces.

The way to correct it is a concept layout showing the developable area, the access, the drainage strategy and the unit count, with a short note explaining why the standard assumption understates the site. That is a modest piece of work and it is the single most effective intervention available at this stage.

Windfall and small site allowances

A windfall allowance is an estimate of homes expected on sites not specifically identified, included in the supply where there is compelling evidence that such sites have consistently come forward and will continue to.

The evidence has to be historic completions data on unidentified sites, with any garden land excluded from the calculation. An allowance asserted without that record is a common ground of objection at examination, because it inflates the supply without identifying where the homes will be.

Small sites matter separately. National policy expects a proportion of the housing requirement to be accommodated on sites of a modest size, and authorities often address that through a small sites allowance or a dedicated allocation exercise. For a promoter with land of under a hectare, that policy is the one to read first, since it may offer a route that does not depend on a strategic allocation at all.

Challenging an assessment outcome

The assessment is evidence, so it is challenged with better evidence rather than with argument.

Four routes, in order of usefulness. Write to the planning policy team when the assessment is published, with the specific entry, the error and the document that corrects it; factual corrections are often simply accepted. Make representations at the draft plan and publication consultation stages, under the local planning regulations, attaching the evidence. Appear at the examination hearing on the relevant matter, where supply and site selection are usually programmed as separate sessions. And where a planning application is the live question, submit the evidence with the application and explain why the assessment entry is out of date.

Keep the correction short and documentary. An officer reviewing hundreds of entries will accept a drawing that resolves an access query far more readily than a letter asserting that the site is suitable. Where a neighbourhood plan also covers the land, check it at the same time, because the question of which neighbourhood plan applies can change which policies the site is judged against.

Enter your postcode on PlanScope to see which reports a site is likely to need once it is through the assessment. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.