When the new 20% housing land supply buffer took effect on 1 July 2026, its immediate significance appeared relatively straightforward. Some local planning authorities that had previously been able to demonstrate a five-year housing land supply would suddenly have to clear a higher threshold. For authorities already close to the margin, that could be enough to change their housing supply position without a single site falling out of the trajectory.

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The publication of the new National Planning Policy Framework on 17 August 2026 has made the position more interesting, not less.

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The buffer remains. What has changed is the decision-making framework around it.

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Under the previous NPPF, the significance of losing a five-year housing land supply was closely associated with paragraph 11(d) and what became known as the “tilted balance”. That familiar formulation has now gone. The August 2026 Framework introduces a different structure of national decision-making policies and a permanent presumption in favour of sustainable development. Housing land supply remains highly relevant, but it now feeds into that structure in a different way.

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That distinction matters because it would be easy to conclude that the disappearance of the tilted balance has made five-year supply less important. In practice, developers and landowners should be wary of reaching that conclusion.

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The basic supply requirement remains familiar. Local planning authorities are expected to identify and update a supply of specific deliverable sites sufficient to provide at least five years’ worth of housing, assessed against the relevant housing requirement or, where applicable, current local housing need. An appropriate buffer is then added to that requirement.

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The normal buffer is 5%. It rises to 20% where housing delivery over the preceding three years has fallen below 85% of the requirement. The additional route introduced from 1 July 2026 also remains: for decision-making purposes, a 20% buffer applies to certain authorities with a housing requirement adopted in the previous five years and examined against a pre-December 2024 version of the Framework, where the annual average requirement is 80% or less of the authority’s most up-to-date local housing need calculated using the standard method.

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This is not the same as replacing an adopted housing requirement with the latest standard method figure. Where the adopted requirement remains the correct basis for the five-year calculation, it continues to perform that role. The current local housing need figure is instead used as a comparator in deciding whether the larger buffer should apply.

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That apparently technical distinction has a significant practical effect.

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A buffer does not increase the number of homes that must ultimately be provided over the plan period. Its purpose is to bring additional supply forward, increasing the amount of deliverable land that an authority must be able to demonstrate in the immediate five-year period. Applying a 20% buffer therefore substantially raises the evidential hurdle. In simple terms, an authority whose supply was comfortably above five years under a 5% buffer may have a materially different position once 20% is applied.

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This makes the calculation itself an obvious point of scrutiny, but the August Framework adds another dimension.

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Policy S5 now expressly recognises housing development responding to an evidenced unmet need as a form of development that may be acceptable in principle outside settlements, subject to the policy's other requirements. An inability to demonstrate a five-year supply of deliverable housing sites is specifically identified as one circumstance capable of evidencing that unmet need. A Housing Delivery Test result below 75% does the same.

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The consequences of housing supply failure have therefore not disappeared with the tilted balance. They have been recast.

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That may prove especially important for sites on the edge of settlements. The old debate often began with whether the tilted balance was engaged and what weight should consequently be given to restrictive development plan policies. The new question is potentially more direct: does the authority have an evidenced unmet housing need, and if so does the proposal satisfy the national policy tests that determine whether development outside the settlement is acceptable in principle?

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This does not make a deficient five-year supply a route to automatic permission. Location, scale, infrastructure, design, heritage, landscape, flood risk, Green Belt and other relevant policy considerations remain capable of being decisive. Policy S5 itself imposes requirements on development seeking to meet unmet need outside settlements. Planning judgement remains essential.

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But the policy significance of the supply calculation is still capable of changing the prospects of a site.

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That is why the 20% buffer deserves more attention than its rather technical language suggests. The immediate question for an authority is whether the additional buffer causes it to fall below five years. The more commercially important question for a developer or landowner is what that failure then means for sites which have previously sat outside the authority's preferred development strategy.

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The 2025 Housing Delivery Test results, published alongside the new Framework on 17 August, reinforce that point. Where delivery is below 85%, the 20% buffer applies. Where delivery falls below 75%, the Framework additionally treats an evidenced unmet need for housing as existing for the purposes of Policy S5. Housing delivery performance, five-year supply and the acceptability in principle of some housing proposals are therefore now connected through a clearer chain of national policy.

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For developers, this means that looking at an authority's headline claim of “five years' supply” is unlikely to be enough.

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The starting point should be the requirement against which supply has been assessed. That should be followed by the applicable buffer, the latest Housing Delivery Test result and, critically, the evidence relied upon to show that the claimed sites are genuinely deliverable within five years.

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The latter point can be just as important as the arithmetic. Five-year supply has always depended on deliverable sites rather than simply allocated capacity. A trajectory can look healthy on paper while being vulnerable to challenge because of infrastructure, ownership, planning status, lead-in times or other barriers to delivery. A larger buffer magnifies that vulnerability because more demonstrably deliverable supply has to be found within the same immediate period.

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Recently published housing land supply statements therefore warrant particular care. A statement prepared before the 20% provision took effect may no longer answer the right question. Even an assessment produced after July needs to be considered alongside the August 2026 Framework and the latest Housing Delivery Test results.

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The practical lesson is not that every authority affected by the new buffer has suddenly lost its five-year supply, nor that every site outside a settlement has become development land. It is that assumptions based on the previous policy regime have a much shorter shelf life than they did a few months ago.

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For landowners considering whether to promote a site, developers assessing acquisition opportunities and applicants deciding whether to submit or appeal, housing supply should therefore be tested early rather than treated as a point to revisit once an application is already in difficulty.

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In some areas the exercise will confirm that little has changed. In others, the difference between a 5% and 20% buffer, a marginal deliverability assumption or a Housing Delivery Test result below 75% may materially alter the planning strategy.

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The terminology has changed since the 20% buffer first arrived. The commercial principle has not. Housing land supply remains one of the points at which national housing policy translates directly into the prospects of individual sites.

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The mistake now would be to look for the old tilted balance and conclude that, because it is no longer there, the supply position matters less. Under the August 2026 Framework, the better question is what the authority's current supply and delivery position evidences about unmet need, and what that means for the site in front of you.