Windfall sites: what they are and why the figure matters
A windfall site is one the development plan never identified. The allowance built on top of them is where the arguments happen, and it is usually the softest number in a supply calculation.
A windfall site in planning is a site that comes forward for development without having been identified in the development plan. No allocation, no site reference, no entry in the plan's housing trajectory. National policy defines them in exactly those terms, and the definition is simple enough that most arguments about windfall sites are really arguments about the allowance built on top of them.
That allowance is a number a local planning authority adds to its housing land supply to reflect sites it expects to come forward without being named. It is contested at almost every local plan examination and at a good many appeals, because it can be the difference between a council demonstrating a five year supply and failing to.
What makes a site a windfall
Three characteristics, in practice. The site is not allocated in the development plan. It has become available unexpectedly, usually because an owner decided to sell or a use became redundant. And it delivers housing that the plan did not specifically plan for.
Typical examples are a redundant garage block on a housing estate, a corner shop converted to flats, an infill plot between two houses, a small brownfield yard behind a high street, or an office building converted under permitted development rights.
What a windfall site is not is a site the plan identified and then failed to deliver. A stalled allocation is a delivery problem, not a windfall, and treating it as one is double counting.
How an allowance enters the housing supply
A five year housing land supply calculation is a list of deliverable sites, mostly allocations and extant permissions, set against the housing requirement. A windfall allowance is a block of units added to that list without naming the sites they will come from.
The mechanics matter. Sites with planning permission are already counted individually, so an allowance covering the same period would count them twice. Authorities normally avoid that by starting the allowance in the third year of the five year period, on the basis that the first two years are already covered by permissions in the pipeline. Where an allowance runs from year one, that is the first thing to test.
The scale of the allowance varies enormously between authorities. A tightly constrained urban borough with a long history of small site delivery can justify a substantial figure. A rural district with a handful of windfall completions a year cannot.
The evidence required to justify one
National policy sets the test plainly. An allowance may be made where there is compelling evidence that windfall sites have consistently become available in the local area and will continue to provide a reliable source of supply. The allowance should be realistic, having regard to the strategic housing land availability assessment, historic windfall delivery rates and expected future trends.
Three words in that test do the work.
Evidence at that standard means more than a trend line. It means a dataset of past completions on non-allocated sites, over a period long enough to cover a market cycle, disaggregated so that the reader can see what kind of sites delivered and at what rate.
Consistently means the delivery has to be steady rather than driven by a single unrepeatable event. An authority whose windfall record is dominated by three large office conversions in one year has evidence of an event, not a trend.
Reliable means forward looking. Past delivery driven by a permitted development route that has since been restricted, or by a supply of redundant buildings that has now been exhausted, does not evidence future supply.
The detail of how authorities build that evidence is set out in our note on how windfall allowances are justified.
Garden land and the policy position
National policy states that a windfall allowance should not include residential gardens. That exclusion follows from the treatment of gardens in built-up areas, which are not previously developed land.
The exclusion is narrower than people assume. It applies to the allowance, not to individual applications. A garden plot can still be developed, and a permission on garden land still counts in the supply as an extant permission. What the authority cannot do is build a forward allowance on the assumption that gardens will keep coming forward.
When testing a published allowance, ask how the historic completions were categorised. Where garden land completions sit inside the historic dataset and the trend has been projected forward from it, the exclusion has been breached indirectly.
Why the allowance is often contested
Because the number is large enough to matter and soft enough to argue about.
The recurring challenges are these. The historic period chosen is too short, or begins at a convenient point. The dataset includes site types that will not recur. The allowance overlaps with sites already counted as permissions. Garden land has not been stripped out. The trend has been projected flat when the supply of the relevant building type is declining. And the allowance has not been revisited since the plan was adopted, though the evidence behind it has aged.
None of those are technical quibbles. Each of them changes the number, and in a five year supply argument the number is the argument.
Using or challenging a published allowance
For an applicant arguing that a council cannot demonstrate a five year supply, the windfall allowance is usually the softest target in the calculation, and it is worth more attention than the deliverability of individual allocations.
Work through it in this order:
- Find the allowance and its evidence base, usually in the authority's housing land supply statement or the plan's evidence library.
- Establish the historic period used and the annual figures within it.
- Check whether the allowance starts in year one or year three.
- Identify the site types making up the historic record and ask whether each remains available.
- Check for garden land in the dataset.
- Test the forward projection against any policy change that affected the supply route.
Set the analysis out in the Planning Statement with the figures in a table, and be specific about how much of the allowance you say is unjustified. A submission that says the allowance is unreliable without quantifying it gives the decision maker nothing to work with.
For an authority, the same list is a checklist for defending the figure.
Effect on a five year supply argument
A windfall allowance that fails scrutiny reduces the supply, and where that takes the authority below the required figure, the consequences flow from the presumption in national policy for decision taking where relevant policies are out of date.
The exact mechanics of the supply requirement, the buffers applied and the consequences of a shortfall have changed between editions of the Framework, most recently in the December 2024 and August 2026 revisions. Check the current edition and state which one you are working to, because a statement of case citing a superseded formulation loses credibility quickly.
One honest caveat. Winning the argument on windfall rarely wins the appeal on its own. It opens the policy door; the scheme still has to survive the balancing exercise on its own merits.
Browse the 66 report types to see which supporting documents carry a housing supply argument, or start from the home page to see what a site needs.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is a windfall site?
A site that comes forward for development without having been specifically identified in the development plan. National policy defines them in those terms. Typical examples include redundant garage blocks, infill plots, small brownfield yards behind high streets, and buildings converted to housing under permitted development rights. A stalled allocation is not a windfall, because the plan identified it.
What evidence justifies a windfall allowance?
National policy requires evidence of a high standard showing that windfall sites have consistently become available in the local area and will continue to provide a reliable source of supply. In practice that means a disaggregated record of past completions on non-allocated sites over a period long enough to cover a market cycle, plus a forward view that accounts for changes in the supply route.
Can garden land be included in a windfall allowance?
No. National policy states that an allowance should not include residential gardens. The exclusion applies to the forward allowance rather than to individual applications, so a permission on garden land still counts in the supply as an extant permission. When testing an allowance, check whether garden completions sit inside the historic dataset that was projected forward.
Why do allowances often start in year three?
To avoid double counting. Sites with planning permission are already listed individually in the five year supply, and an allowance covering the same period would count the same units twice. Starting the allowance in the third year reflects the assumption that the first two years are covered by the existing permissions pipeline. An allowance running from year one is worth testing.
How much difference does the windfall allowance make?
Enough to decide whether an authority can demonstrate a five year supply. The allowance is often one of the larger single components of the calculation and rests on the softest evidence, which is why it attracts more challenge than the deliverability of individual allocations. Quantify the part you say is unjustified rather than asserting the figure is unreliable.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.