Windfall sites: how councils count unplanned homes in supply

A windfall site is a site that comes forward for housing without having been identified in the development plan. The National Planning Policy Framework allows a local planning authority to include an allowance for windfall sites in its housing land supply, but only where there is compelling evidence that such sites have consistently become available and will continue to provide a reliable source of supply. That evidential bar is what appeals turn on, because an authority that inflates its windfall allowance can appear to have a supply it does not have.

For anyone arguing about housing land supply, the windfall line in a five year supply statement deserves as much attention as the largest allocation. It is often less well evidenced.

What makes a site a windfall

The definition is about the plan, not the site. If a site was allocated, or otherwise specifically identified as available through the plan-making process, homes delivered on it are planned supply. If it was not, and it comes forward anyway, it is a windfall.

Typical windfalls include:

• Small infill plots and garden land, though gardens are treated separately below.
• Conversions of upper floors above shops.
• Former commercial premises in town centres.
• Prior approval conversions, such as office to residential changes.
• Redevelopment of a site nobody put forward during the call for sites.

The category is wide, and that is part of the problem. An authority counting all of these together can produce a trend line that looks stable while its components change completely.

Prior approval conversions are the clearest illustration. A borough that saw a burst of office to residential changes in one three year window will carry that burst in its historic average for a decade, long after the stock of convertible offices has gone. The trend looks reliable. The underlying source has closed.

Why the allowance matters to the supply calculation

Housing land supply is a comparison between a requirement and a deliverable supply. Windfall allowances sit on the supply side, usually as an annual figure applied to the later years of the period on the reasoning that the earliest years are already covered by sites with permission.

The effect is arithmetic and blunt. An allowance of 100 homes a year over three years adds 300 homes to supply, which on a requirement of 500 a year moves the calculation by roughly half a year of supply. Where the authority is close to the threshold, the windfall line alone can decide whether the tilted balance applies to your application. That is an illustrative calculation rather than a real authority's figures.

The Framework's edition matters here. Verify the current policy wording and code on gov.uk, because the Framework was reissued on 17 August 2026 with policy coded chapters in place of the paragraph numbers used in the December 2024 and December 2023 editions, and older appeal decisions quote the old numbering.

The evidence requirement behind an allowance

The test means more than a table of past completions. Read the authority's evidence for four things.

1. The length and consistency of the historic trend, and whether the years counted include an unusual spike such as a wave of office to residential prior approvals.
2. Whether the sources that produced past windfalls still exist. A town that has already converted most of its upper floors cannot keep converting them.
3. Whether the allowance double counts sites that appear elsewhere in the supply, particularly small sites with permission.
4. Whether the strategic housing land availability assessment already identifies the sites now being claimed as unidentified windfalls.

The last point is the one that most often unravels an allowance. A site cannot be both specifically identified in the assessment and an unplanned windfall.

Where an authority relies on a policy change to justify a higher future rate, ask for the quantified basis. A general expectation of more small site delivery is an aspiration.

Residential gardens

The Framework's windfall policy excludes residential gardens from any windfall allowance. Authorities sometimes comply on paper while including garden derived completions in the historic trend used to set the allowance, which produces the same result by a different route.

Check the underlying completions data. If the trend that supports the allowance is built partly on garden plots, the forward allowance inherits the problem, and it is a fair point to make in representations or at an appeal.

Challenging an inflated allowance

Build the challenge on the authority's own published documents rather than on assertion. The authority monitoring report, the five year supply statement, the strategic housing land availability assessment and the local plan examination evidence base are all public, and they frequently disagree with each other.

A workable structure for the argument:

• State the allowance claimed and the years it applies to.
• Set out the historic completions the authority relies on, with the source and the date of publication.
• Identify the components of that historic figure that will not repeat, with reasons.
• Identify any double counting against sites listed elsewhere in the supply.
• Recalculate the supply with an adjusted allowance and state the resulting position.

Present it as arithmetic. Inspectors respond to a table with sourced figures far better than to adjectives, and the same discipline applies whether you are arguing about windfalls, about the openness of the Green Belt or about any other contested policy test.

Windfalls in the housing delivery test

The Housing Delivery Test measures homes actually delivered against homes required, so windfall completions count towards delivery in exactly the same way as allocated sites. An authority can therefore pass the delivery test on the back of windfalls while its plan-led supply underperforms.

That is a useful point in a plan-making context and a limited one in development management, because the delivery test looks backwards at what was built rather than forwards at what is deliverable.

Using the published trend data

If you are promoting a site, the same data works in your favour. An authority whose windfall allowance is doing heavy lifting is an authority whose plan-led supply is thin, and that is the argument to make in the planning statement supporting the application.

Pull the numbers from the authority monitoring report for each of the last ten years, split them by source where the report allows it, and plot them. The shape of that line, rather than the average, is what tells you whether the allowance is defensible. A declining trend with a single spike is a different proposition from a flat one. Ten years is the minimum useful run; five years starting in an unusual year tells you almost nothing, and authorities know which start date flatters them.

A short note on tone. Windfall arguments are technical and they are rarely decisive on their own, so they work best as part of a wider supply case rather than as the headline. Pair the analysis with the substantive planning merits, and use the report types to structure the supporting documents. To scope the pack for a promoted site, start from your postcode on the PlanScope home page.

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