The 20% buffer is live: what a weaker five-year land supply means for your fields

On 1 July 2026 a small change to national planning policy took effect that will not have made the news, but that materially changes the odds on a great many sites. The December 2024 revision of the National Planning Policy Framework, published on GOV.UK, raised the buffer that certain councils must add to their five-year housing land supply from 5% to 20%. It applies to authorities whose adopted local plan is delivering less than 80% of the housing need calculated under the government’s standard method. In blunt terms, the councils that are furthest behind now have to find a fifth more deliverable land than before, and many of them cannot.

This is a plain-English translation of a technical rule, not legal advice, and the exact position for any one council is nuanced and moves quickly. Treat what follows as the shape of the thing, and check the current picture for your own authority before you act on it.

What the buffer actually does

Every local planning authority is expected to demonstrate a deliverable supply of housing land measured against its need, the familiar five-year housing land supply. The buffer is a margin added on top, a deliberate over-provision so that the pipeline does not run dry if some sites stall. Raising it from 5% to 20% sounds modest, and it is anything but. That jump is the difference between a council that could just about show five years of supply and one that suddenly cannot.

The trigger matters as much as the number. The 20% buffer bites where a council is delivering under 80% of its standard-method need, so it targets exactly the authorities that have been building too slowly. For those places the bar has been lifted at the moment they were least able to clear it. There is one wrinkle worth naming, reported by Planning Resource: some authorities caught by transitional arrangements around the plan-making reforms face a six-year supply requirement rather than five, which is a steeper test again. The rules are not uniform, and the precise obligation depends on where a council sits in the transition.

Why a shortfall opens a window

When a council cannot demonstrate the required supply, the presumption in favour of sustainable development in paragraph 11(d) of the NPPF is engaged in its stronger form, the tilted balance. The local plan policies that would normally be used to resist a housing scheme are treated as out of date, and permission should be granted unless the harm of doing so would significantly and demonstrably outweigh the benefits. As firms such as Stephens Scown and commentary in Local Government Lawyer have set out, this is the mechanism that decides a large share of contested housing appeals.

For a landowner the consequence is direct. On an unallocated site next to a town, in a district that is short of supply, the tilted balance can carry a scheme that would have been refused two years earlier. The same field can be a firm no under a council with a healthy supply and a realistic yes under a council that has just been pushed below the line by the new buffer. This is not a loophole. It is national policy working as designed, leaning towards homes where a council has failed to plan for enough of them.

The scale of exposure is real. Urbanist Architecture’s tracker recorded around 157 councils unable to demonstrate a five-year supply as of June 2026, and that count was taken before the 20% buffer applied to the weakest performers on 1 July. The pool of districts where a well-promoted site can win consent outside the current plan is wide, and for the moment it is widening.

The honest limits, and why timing beats price

None of this is a guarantee, and it would be a disservice to suggest otherwise. The tilted balance improves the odds. It does not switch off the planning system, and real constraints still bite hard. Green Belt, functional flood plain and designated landscapes can defeat a scheme regardless of how short a council’s supply is, as can a genuine highways or ecology problem. A tilted balance on the wrong site is still a refusal. It rewards land that was well located and developable to begin with, not land that simply happens to be available.

There is also the matter of the window closing. A supply shortfall is a moment, not a permanent state. Councils fix their position by adopting a new plan, or by allocating more land and getting delivery moving, and when they do the tilted balance switches off. The owners who benefit are the ones who understood their authority’s position early and had a credible scheme ready to go into that window, not the ones who noticed after it had shut. That is why timing a promotion to the shortfall usually matters more than the headline price on any option. A slightly keener price on a site that misses the window is worth far less than a well-timed consent.

Working out whether your own council is exposed is genuine work. There is no single official live feed of every authority’s five-year supply, and the figure is calculated per council and routinely contested at appeal. Our free Land Potential Check will read your land against national planning data and flag the constraints that would still apply whatever the supply position, which is the honest place to start. Where the buffer is leaning your way, it is worth knowing while the door is open. And where a hard constraint means it is not, it is worth knowing that too, before anyone spends money finding out the slow way.