Windfall sites and how allowances are justified

A windfall site is one that comes forward for housing without ever having been identified in the development plan. Windfall sites planning arguments matter because councils are allowed to count an allowance for future windfalls in their housing land supply, and every unit counted that way is a unit that does not need an allocation. Get the allowance wrong and the supply position is wrong with it.

National policy sets a single condition on using an allowance: there must be compelling evidence that windfall sites will provide a reliable source of supply. That evidential bar is the hook for every challenge, and most disputes at examination and at appeal turn on whether the evidence reaches it.

What a windfall site is

The definition is simple and its edges are not. A windfall is a site that has not been specifically identified as available in the development plan process, which usually means it is not an allocation and did not appear in the land availability assessment.

In practice the category is dominated by small urban sites: infill plots, backland development, conversions of upper floors, redundant garage courts, subdivided plots, and buildings converted to residential under prior approval rights. Larger windfalls happen, but they are irregular by nature, which is exactly why an allowance based on them is hard to justify.

Two boundary questions recur. Sites that appear in a brownfield land register but are not allocated are still windfalls for supply purposes, though their appearance on the register is useful evidence of availability. And prior approval conversions under permitted development rights are windfalls, since nobody allocated them, which matters because their rate of delivery has moved sharply with successive changes to the rights themselves.

Why allowances appear in housing supply

A council has to demonstrate a supply of deliverable housing sites against its requirement. It builds that supply from sites with permission, allocations, and any windfall allowance it can justify.

The attraction of an allowance is obvious. It adds units without identifying land, which means without the local political cost of allocating a site somebody objects to. A generous allowance in the later years of the plan period can make the difference between a plan that demonstrates supply and one that does not.

That is why promoters test them. If an allowance of a few hundred units a year is removed at examination, the council either allocates more land or fails the test, and either outcome opens the plan up.

The same argument runs at appeal, where the question is whether the authority can demonstrate the required supply at the date of the decision. An inflated windfall allowance can be the difference between a decision taken under the ordinary policies of the plan and one taken with the balance tilted in favour of permission.

The evidence needed to include one

Meeting that bar means historic data, analysed, with an explanation of why the past predicts the future.

The starting point is a run of past completions on unidentified sites, ideally covering ten years or more so that the series spans a full economic cycle. A three year run taken from the recovery after a downturn is not evidence of a trend, it is evidence of a recovery.

The analysis then has to strip out what will not recur. Sites already counted elsewhere in the supply. Large one-off releases that inflate a single year. Conversions delivered under a permitted development right that has since been narrowed. Prior approvals granted under rights that have since been withdrawn by an Article 4 direction, which many authorities have made for office conversions in their town centres.

Finally the projection has to explain the future trend. If the historic rate depended on a stock of redundant offices that has now been converted, the same rate cannot continue. If a new right has opened up a source that did not exist in the historic period, the council needs to say how much it will deliver rather than assuming continuity.

An allowance that survives scrutiny usually has four features: a long data series, a clear method for excluding non-recurring sources, a lower rate in the early years of the plan period where sites with permission are already counted, and a sensitivity test.

Residential gardens and their treatment

National policy is explicit that a windfall allowance should not include residential gardens, and that exclusion has consequences for the arithmetic.

Garden land was a substantial source of small-site completions in many suburban districts before policy turned against it. A historic series that includes garden development therefore overstates the future rate unless the garden component is identified and removed, and identifying it requires the council to look at the planning history of each site rather than a headline count.

Ask for the disaggregation. Where the council cannot say how many of its historic windfalls were garden sites, the allowance is built on a number it cannot explain, and that is a straightforward point to make at examination.

The related question is whether the authority has a policy restricting garden development. Where it does, the historic rate will fall further, and the projection should show it.

Challenging an inflated allowance

The effective challenge is arithmetic rather than rhetoric. Work through the council's own data and identify the specific components that will not repeat.

Double counting is the most common error. Sites with planning permission are counted individually in the supply, and if the windfall allowance runs from year one it may count the same sites again. A properly constructed allowance starts in year three or later for that reason, and an allowance running from year one deserves a question.

The second is definitional drift. Some authorities count sites that were identified in the land availability assessment but not allocated as windfalls, which stretches the definition beyond what the policy contemplates.

The third is optimism about lead-in times. Small sites are often assumed to build out within a year of permission, and the evidence in many districts does not support that.

Where you are promoting a site, the Planning Statement should carry the supply argument with the workings attached rather than a bare assertion that supply is not demonstrated. Inspectors respond to tables.

How allowances are tested at examination

At local plan examination the inspector tests whether the plan is positively prepared, justified, effective and consistent with national policy. A windfall allowance is examined under justified, meaning proportionate evidence, and under effective, meaning deliverable over the plan period.

Expect the inspector to ask for the raw data, the exclusions applied, and the reasoning behind the annual rate. Expect them to reduce an allowance rather than remove it entirely where the evidence supports some allowance but not the figure claimed.

Objectors get the most traction by proposing an alternative figure derived from the council's own data, rather than arguing that no allowance is justified at all. An inspector faced with two numbers and one method usually adopts the method.

Practical effect on a site promoter

For anyone promoting land, the windfall allowance is one of the levers that determines whether there is a gap in supply for their site to fill.

Three practical steps. Read the housing land supply statement and the topic paper behind it, not just the headline figure. Rebuild the windfall calculation yourself using the council's completions data, which is published. And raise the point in writing at the earliest consultation stage, because a challenge first made at examination looks opportunistic even when it is right.

Where the site is outside a settlement boundary and the supply argument is the route to permission, the same discipline applies to every other component of the supply. A rural exception site follows a different policy route entirely, and the unit size policy the council applies will shape what a windfall site can actually deliver.

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. Browse the report index to see which documents a housing application needs.