Windfall sites and the housing supply calculation

A windfall site is one that comes forward for housing without having been identified in the development plan: an infill plot, a redundant garage court, a yard behind a parade of shops, a farm building conversion. Windfall sites housing land supply arithmetic matters because authorities are allowed to count an allowance for sites like these in their projected supply, and that allowance can be the difference between a council demonstrating a five year supply and not demonstrating one. If you are arguing that an authority cannot meet its supply, the windfall allowance is usually the softest part of its case.

The rule is short. The evidence it demands is not.

What counts as a windfall

The defining feature is absence from the plan. A site allocated in the local plan is not a windfall even if it comes forward unexpectedly early, and a site identified in the housing and economic land availability assessment and counted individually in the supply is not a windfall either, because counting it twice would double the number.

Windfall delivery is dominated by small sites. Conversions, subdivisions, upper floors over shops, single plots in village envelopes and the redevelopment of small brownfield parcels. It is real supply, and in many authorities it has delivered steadily for decades.

One category is explicitly excluded. The National Planning Policy Framework, in the August 2026 edition as in its predecessors, says a windfall allowance should not include residential gardens. That exclusion exists because garden development was politically contentious and because counting gardens encourages authorities to rely on a source they have no control over. An allowance built on garden land also sits badly with the design and character policies most of those same authorities apply when the applications arrive.

Why an allowance needs more than an assertion

National policy permits a windfall allowance in the five year supply only where there is compelling evidence that such sites will continue to provide a reliable source of supply. The allowance should be realistic having regard to the land availability assessment, historic windfall delivery rates and expected future trends.

Three words in that formulation do the work. The first sets a bar higher than merely reasonable. Reliable asks about the future rather than the past. And trends invites an argument about whether the past still predicts anything at all.

The practical effect is that an authority cannot simply assert a number. It has to show its working, and the working is auditable.

Historic completion rates as the usual evidence

The standard evidence is a run of monitoring data showing windfall completions year by year, usually over ten years or more, with allocated sites and garden sites stripped out.

Four questions decide whether that evidence survives scrutiny.

Does the data actually exclude gardens, or does it exclude only sites recorded as gardens in a monitoring system that never coded them. Many authorities cannot separate the two retrospectively, which is a real weakness in their case.

Does the period include an unrepeatable source. A decade in which a hospital site, a series of redundant depots and a run of office conversions all came forward is not evidence that similar sites remain. Strip those out and the underlying rate is often half the headline average.

Have the policy conditions changed. Office to residential conversions under permitted development rights produced a large volume of windfall completions in some authorities, and a change to those rights or an Article 4 direction removing them alters the forecast completely.

Is the trend flat, rising or falling. An authority taking the ten year average when the last four years are well below it is choosing a flattering number.

Where the allowance sits in the supply calculation

The allowance is added to the deliverable supply alongside sites with permission and allocated sites. Because it is a projection rather than a list of sites, it carries no planning status and nobody can point to where the homes will be.

Most authorities avoid counting windfall in the first year or two of the five year period, on the basis that anything deliverable within that window already has permission and is counted individually. An allowance applied across all five years usually double counts the early years, and that is a straightforward point to make.

The number itself is normally expressed as dwellings per year. Multiply it out before you argue about it. An allowance that sounds modest annually becomes a large share of the five year requirement, and on a small authority it can exceed an entire strategic allocation in the same plan.

Challenging an authority's assumption

If the shortfall you need to demonstrate is smaller than the windfall allowance, start here rather than with the delivery assumptions on individual sites, which are slower to argue and better documented.

1. Get the authority's housing land supply statement and the monitoring data behind the allowance, which should be published.
2. Rebuild the historic series yourself and check what has been included, particularly gardens and permitted development conversions.
3. Identify any unrepeatable source within the period and recalculate without it.
4. Test whether the allowance is applied in years one and two, and whether those homes are also counted as permissions.
5. Compare the allowance with the most recent three years of actual windfall delivery rather than with the long-run average.

An inspector will not accept a bare assertion that the allowance is too high, and a submission that only asserts it wastes the opportunity. The argument has to be arithmetical and it has to use the authority's own published figures. Where the plan itself is old, the related question of which policies still carry weight is covered in our note on why old local plan policies still apply.

Using the point in an application

For an applicant, the windfall argument is rarely the whole case. It is one input into whether the authority can demonstrate a five year supply, and the consequence of a shortfall is the engagement of the policy balance that favours granting permission for sustainable development.

Write it carefully in the Planning Statement. Set out the authority's published position, the specific components you challenge, the arithmetic, and the revised figure, then state what follows from it rather than leaving the reader to infer. Attach the data.

Two cautions. Supply positions move, sometimes within weeks, as monitoring updates and appeal decisions land, so date your analysis and say what it is based on. And appeal decisions on supply in the same authority are indicative of how inspectors have approached the question rather than binding, so use them as argument rather than as proof.

There is a real tension in this area that nobody has resolved. Windfall delivery really is unpredictable, and an authority that refuses to count it understates its supply while one that counts it generously overstates it. Inspectors are left making a judgement on evidence that is always partly historic.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and confirm the current Framework wording, since the Framework has been re-issued more than once in recent years.

Browse the 66 report types to see which documents support a housing application where supply is in dispute.

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