Windfall sites and their role in housing supply

A windfall site is one that comes forward for housing without having been identified in the development plan. Nobody allocated it, no site assessment scored it, and in most authorities these sites deliver a steady proportion of completions every year: a pub converted to flats, a redundant garage yard, a builder's merchant that closed, a single plot in a back garden.

The argument that matters is not what a windfall is. It is whether a council may count future windfalls in its housing land supply before they exist, and how much of an allowance the evidence supports. That question decides five-year supply cases, and five-year supply cases decide appeals.

What a windfall site is

National policy defines windfall sites as sites not specifically identified in the development plan. The definition is negative, which is why the category is so broad. It catches previously developed land and greenfield, urban infill and rural exception sites, large schemes and single dwellings.

Two distinctions are worth keeping straight. A windfall is not the same as a small site, although most windfalls are small. And a windfall is not the same as an unallocated site that the plan positively supports through a criteria-based policy, because that site is being planned for even if it is not drawn on the map.

The practical consequence is that a windfall allowance is a forecast of planning decisions that have not been made, made by the authority that will make them.

Why an allowance is permitted at all

The Framework allows a windfall allowance in the five-year housing land supply where there is compelling evidence that such sites will provide a reliable source of supply. The current Framework was published on 17 August 2026 and is organised by policy code rather than by paragraph number, and it was preceded by the December 2024 edition, which itself renumbered the December 2023 text. Verify the wording and the policy reference in the edition you are working from, because submissions that cite the wrong edition get picked apart.

The logic behind the allowance is straightforward. If an authority has delivered a consistent volume of unallocated completions for a decade, and nothing has changed that would stop it happening, ignoring that supply would understate the position. The allowance exists to avoid that understatement, not to fill a gap in allocations.

That distinction is where most disputes start.

The evidence needed to justify an allowance

That test calls for a trend, an explanation and a forward look. A defensible allowance normally rests on:

1. Historic completions on windfall sites over a long enough period to cover a market cycle, usually ten years or more, taken from the authority's own monitoring.
2. A breakdown by source, so it is clear whether the past supply came from office conversions, garden plots, redundant employment land or rural conversions.
3. An assessment of whether each source will continue, including any policy or market change that has closed it off.
4. A forward-looking adjustment where a source has been exhausted or a national change has altered its likelihood.
5. A statement of how double counting with allocations and with extant permissions has been avoided.

Point three is where allowances fall over. A windfall record built mainly on permitted development conversions from one use to residential is not a record of general market behaviour. If the stock of convertible buildings in the area is largely used up, the historic rate says little about the next five years.

Point four cuts both ways. Authorities are usually reluctant to reduce an allowance, and objectors are usually reluctant to accept that a new policy could increase one.

Double counting against allocations

The commonest technical error is counting the same homes twice. It happens in three ways.

A site that is allocated cannot also be a windfall, so any allowance must exclude allocated sites even where they came forward unexpectedly early. A site with planning permission is already in the supply as a commitment, so the allowance must apply only to the years in which new, currently unknown sites would come forward. And where the plan includes a criteria-based policy that positively plans for a category of site, counting that category as windfall as well double counts the policy.

The practical test is timing. A windfall allowance should normally be applied to the later years of the five-year period, because sites coming forward in year one already have permission and are therefore commitments. An allowance applied evenly across all five years usually contains double counting, and saying so is often the most productive line of attack on an authority's supply position.

Garden land and category exclusions

Earlier editions of the Framework excluded residential gardens from the windfall allowance, reflecting a policy position that garden development should not be assumed. Check the wording of the edition currently in force rather than relying on that, because the treatment of garden land has been revisited more than once.

Whatever the national position, the local evidence base may exclude categories of its own. An authority that has adopted an Article 4 direction removing permitted development rights for a category of conversion has closed off a source of past windfall supply, and its allowance should reflect that. The same applies where a local plan policy now resists a form of development that historically produced completions.

Challenging an inflated allowance

If you are arguing that an authority cannot demonstrate a five-year supply, the windfall allowance is often the most rewarding component to examine, because it is a forecast rather than a list.

Ask for the monitoring data behind it rather than the summary. Test whether the historic period includes unusual years. Check whether the sources of past supply still exist. Look for double counting in year one and year two. And check whether the allowance was set at plan examination and simply carried forward, unchanged, into a housing land supply statement written years later.

An allowance that was reasonable at adoption can become indefensible by the time the plan is five years old, and authorities rarely revisit them unprompted.

Using the point in an appeal

Present the argument quantitatively. State the allowance the authority claims, identify the component you say is unjustified, quantify the effect on the supply position, and show the resulting figure. Inspectors deal with supply arguments constantly and respond to arithmetic that can be checked.

Keep the policy framing separate from the numbers. Where the reduced supply figure engages the policy that tilts the balance in favour of sustainable development, say so plainly and set out the consequence for the decision, tied to the current Framework edition. The related argument about whether a decision would be premature to an emerging plan runs on a different track, and prematurity and what it means in planning is worth distinguishing carefully in submissions where both are raised.

The supply case belongs in the Planning Statement rather than in a separate document, cross-referenced to the authority's own published position so the inspector can compare the two. Where a scheme depends on the supply argument, put the arithmetic in a table and keep the narrative short.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and verify all policy references against the current published Framework.

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