Windfall allowances in housing land supply

A windfall allowance is a number a local planning authority adds to its housing land supply for sites it cannot yet name. It is the only component of the five year supply that rests entirely on prediction, which is why it attracts more scrutiny at appeal than any other line in the calculation, and why national policy sets a high evidential bar.

Get the evidence right and the allowance survives examination. Get it wrong and an inspector will strike it out, which on a marginal supply position is enough to decide an application.

What counts as a windfall site

A windfall site is one that comes forward for housing without having been specifically identified in the development plan. Infill plots, redundant commercial premises, conversions above shops, prior approval schemes, backland development and small brownfield parcels are the usual sources.

The definition is exclusionary rather than descriptive. If the site is allocated, it is not a windfall. If it already has planning permission and is counted as a deliverable site in the supply, it is not a windfall either, because the supply already includes it by name.

That second point is where double counting creeps in, and it is the first thing to check in any disputed supply position.

The evidence test in national policy

National policy allows an allowance for windfall sites in the five year supply only where there is compelling evidence that such sites have consistently become available and will continue to provide a reliable source of supply. The allowance has to 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. "Consistently" rules out a series with a single exceptional year carrying the average. "Reliable" asks whether the mechanism producing the sites still exists. And "expected future trends" makes the historic rate a starting point rather than an answer.

The policy is expressed in the current National Planning Policy Framework, published on 17 August 2026, and the wording has been carried through successive editions with little change. Check the current text, because the surrounding supply requirements have moved more than the windfall provision itself.

Building the historic series

A defensible allowance starts from a completions series, normally ten years or more, with each year split between allocated and non-allocated sites. Anything shorter risks capturing one part of an economic cycle.

Four things have to be stripped out before the series means anything.

1. Completions on allocated sites, which belong to the allocation rather than to the windfall trend.
2. Residential gardens, which national policy expressly excludes from windfall allowances.
3. Sites already in the current supply with permission, which would otherwise be counted twice.
4. Replacement dwellings and net-to-gross confusion, since a demolition and rebuild of one house for one house is not a net addition.

Present the result as a table with the gross figure, each deduction and the net windfall rate by year. Authorities that publish the workings tend to have their allowances accepted. Authorities that publish a single averaged figure invite an unpicking exercise at every appeal.

Residential gardens are out

The exclusion of residential gardens is absolute in the policy wording and it is frequently overlooked in the underlying data, because monitoring systems do not always distinguish a garden plot from other small sites.

Where the authority's own records do not identify garden land, the deduction has to be estimated, and that estimate should be stated as an assumption. An appellant who obtains the completions list and identifies garden plots that were counted has a strong point, and it is one of the more productive lines of enquiry in a supply argument.

The exclusion does not prevent garden development happening. It prevents the authority relying on it as a predicted future source of supply, which is a different thing and reflects the policy encouraging authorities to protect gardens through their own plan policies.

Which years the allowance applies to

Standard practice applies the windfall allowance only to the later years of the five year period, commonly from year three onwards. The reason is arithmetic rather than policy. Windfall sites that will complete in the next two years mostly already have planning permission, so they are already in the supply by name, and adding an allowance on top counts them twice.

An authority applying a full allowance across all five years should be asked to explain how it has avoided that overlap. Frequently the answer is that it has not.

The same logic applies in reverse when a plan is being examined for a longer period, where the allowance may run across the plan period from a later start date.

When the past stops predicting the future

The reliability test is about mechanism. Ask what produced the windfall sites in the historic series, and whether that mechanism still operates.

Several things break it. An Article 4 direction removing a permitted development right that had been generating conversions changes the pipeline immediately. A new local plan that allocates the kind of sites that were previously coming forward as windfalls converts future windfalls into allocations. Exhaustion of a finite stock, such as redundant town centre upper floors, shows up as a declining trend rather than a flat one. And a change in national permitted development rights can move the rate sharply in either direction, which is one reason written ministerial statements are worth tracking in supply work.

Constraint changes matter too. A nutrient neutrality requirement across a catchment can stop small sites coming forward almost entirely, because the mitigation cost falls hardest on schemes with no economies of scale.

Where the authority's settlement strategy has changed, the geography of future windfalls changes with it, and settlement boundaries in local plans are the mechanism that determines where infill can occur at all.

Challenging or defending the number

Work through the same checklist from either side.

• Ask for the completions data behind the allowance, by site, by year, with the allocation status of each.
• Check whether gardens have been excluded, and how that exclusion was identified.
• Check the treatment of permissions already in the supply, and which years the allowance is applied to.
• Look at the shape of the series rather than the mean: a declining trend with a flat allowance is the most common weakness.
• Identify policy or constraint changes since the series began, in both directions.
• Compare the allowance against the land availability assessment, which should corroborate the small sites capacity.

Then argue the point in proportion. A windfall allowance is rarely the largest component of a supply position, and an appeal that stakes everything on it while conceding the deliverability of the strategic sites has picked the wrong fight. It is most valuable where the authority's claimed supply sits just above the requirement, and where removing or halving the allowance takes it below.

For plan making rather than decision taking, the evidence sits alongside the wider tests a plan has to pass, including the duty to cooperate, and an allowance that fails at examination usually fails because it was never shown as a calculation.

A Planning Statement arguing a supply case should set the windfall analysis out as a table with the source of each figure, rather than as narrative. Inspectors read tables.

Browse all 66 PlanScope report types and start a draft to see how a supply argument fits with the rest of an application.

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