Housing trajectory and the five year buffer
The buffer is applied to the requirement, not the supply, so it moves the answer more than almost any other input. How the calculation works and the six points where it is usually contested.
A housing trajectory is the year-by-year projection of how many homes an authority expects to be completed across its plan period. The five year housing land supply calculation is a slice through it, comparing deliverable supply against five years of requirement, adjusted for any shortfall and increased by a buffer. Get the buffer wrong and the whole answer moves, which is why the buffer is argued over more than almost any other number in an appeal.
What a trajectory shows
Plans are expected to include a trajectory illustrating the expected rate of delivery over the plan period. In monitoring, the authority updates it annually, usually in an authority monitoring report or a standalone supply statement.
A useful trajectory does three things. It sets out past completions against the requirement, so the reader can see whether delivery has kept pace. It projects future completions site by site, with the assumptions exposed. And it identifies where the supply is concentrated, which is where the risk sits.
A trajectory showing 40 per cent of the five year supply coming from three strategic sites is telling you something important about deliverability, whatever the headline figure says.
The base date and why it decides arguments
Every supply position has a base date, and the choice of date shifts the answer.
Supply is assessed at a point in time, and both the requirement and the deliverable sites are measured from there. A statement with a base date of 1 April, published in December, is eight months out of date before it is read, and completions and permissions since then are legitimately in issue.
Three base date arguments come up repeatedly. Whether sites completed since the base date have been double counted in the supply. Whether permissions granted since the base date should be added. And whether the requirement has moved because the standard method inputs have been updated.
Be consistent. If you add post base date permissions to the supply, you have to add post base date completions to the delivery side as well. Inspectors notice selective updating quickly.
How the buffer works
The buffer is applied to the five year requirement before the supply is divided into it, so a larger buffer produces a larger requirement and a smaller number of years of supply.
National policy has applied a standard buffer to ensure choice and competition in the market for land, with a larger buffer where there has been under-delivery measured through the Housing Delivery Test. Historically those figures have been 5 per cent and 20 per cent.
The buffer regime has been amended in each of the last three revisions of the National Planning Policy Framework, including the December 2024 edition and the edition issued on 17 August 2026, which reorganised the Framework around policy codes rather than paragraph numbers. Check the current levels, the triggers and the wording on gov.uk before you rely on them, and state the edition you are citing in any submission. An appeal statement quoting a superseded paragraph number is an easy point for the other side.
The arithmetic itself is straightforward.
- Establish the annual requirement, from the adopted strategic policy where it remains current, or from local housing need.
- Multiply by five.
- Add or deduct any shortfall or oversupply, applying the chosen method.
- Apply the buffer.
- Divide by five to get the annual requirement against which supply is measured.
- Divide the deliverable supply by that figure to express supply in years.
The delivery test and what it triggers
The Housing Delivery Test measures net homes delivered against homes required over the previous three years, and it is published annually by the government rather than calculated locally.
The consequences have been staged by percentage. An authority delivering below a first threshold has to publish an action plan. Below a second, the larger buffer applies. Below a third, the presumption in favour of sustainable development is engaged, so the policies most important for determining the application are treated as out of date and permission should be granted unless the adverse impacts would significantly and demonstrably outweigh the benefits.
Those thresholds have sat at 95, 85 and 75 per cent for several years. Verify them against the current Framework, because this is another area that has moved.
The interaction between the delivery test and the supply position matters. An authority can fail the delivery test and still demonstrate five years of supply, and it can pass the test and fail on supply. They measure different things, one backward looking and one forward looking, and a submission should address both.
Shortfall and the two methods
Where past delivery has fallen behind the requirement, the shortfall has to be made up. Two approaches are used, and the choice can change the answer by a year or more.
| Method | How it works | Effect |
|---|---|---|
| Sedgefield | The whole shortfall is added to the first five years | Higher requirement, lower years of supply |
| Liverpool | The shortfall is spread across the remaining plan period | Lower requirement, higher years of supply |
National guidance has favoured dealing with shortfall in the first five years where possible. Authorities near the end of a plan period sometimes argue for the spread approach because the remaining period is short and the effect is similar. Applicants generally argue the other way.
Do not treat the choice as settled by precedent in a neighbouring district. It is a matter for the decision maker on the evidence, and the reasoning in one appeal decision is indicative rather than binding.
Where the calculation is usually disputed
Six areas account for most of the argument, and a submission that engages with them specifically is worth more than one that recites the headline figure.
- Lead-in times, meaning the period between permission and the first completion, which authorities tend to underestimate on large sites.
- Build-out rates, particularly the number of outlets assumed on a strategic site.
- Whether sites meet the definition of deliverable, which requires clear evidence for sites without detailed permission.
- Windfall allowances, and whether there is compelling evidence from past delivery to support the rate assumed.
- Sites with unresolved constraints such as access, remediation, nutrient neutrality or utilities capacity.
- Double counting between allocations, permissions and windfall.
Evidence beats assertion. A letter from the housebuilder on a strategic site saying when it expects to start, and at what rate, carries more weight than a consultant's table. So does a planning history showing that a site has been in the supply for six consecutive years without a spade going in.
Where nutrient neutrality or water neutrality affects a catchment, the deliverability of otherwise permitted sites becomes a live question, and the supply argument and the habitats argument have to be run together.
Using the figures in a submission
Lead with the conclusion and show the working. An inspector reading a supply chapter wants to know what you say the figure is, how it differs from the authority's, and which specific inputs create the gap.
Set out a reconciliation table listing each disputed site, the authority's assumed delivery, your assumed delivery and the reason for the difference. Then state the resulting supply figure on your case and on the authority's case, so the inspector can see the range.
Where the tilted balance is engaged, the Planning Statement has to carry the full balancing exercise rather than asserting that a supply shortfall decides the application. It does not. It changes the test.
Affordable housing delivery is a separate but related question, since an authority failing on affordable need may attach weight to a scheme's affordable contribution even where market supply is adequate, and that argument belongs in the Affordable Housing Statement rather than buried in the supply chapter. Where the authority resists on the basis that the application should await the emerging plan, the reasoning set out in prematurity in planning is the answer to have ready.
As an illustrative example, an authority claims 5.2 years. Removing one strategic site with no reserved matters application, correcting a lead-in assumption on a second, and reducing a windfall allowance unsupported by past delivery takes the figure below five. That is three specific points, each evidenced, and it is a stronger case than a general complaint that the supply is optimistic.
To see which supporting documents an application of this kind is likely to need, browse the 66 report types by category or enter your postcode on PlanScope.
This is general guidance, not legal advice, so 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.
Frequently asked questions
What is a housing trajectory?
A year-by-year projection of expected housing completions across a plan period, included in the plan and updated annually in monitoring. A useful trajectory shows past completions against the requirement, projects future delivery site by site with the assumptions exposed, and reveals how far the supply depends on a small number of large sites.
Is the buffer applied to supply or to the requirement?
To the requirement. The buffer increases the five year requirement before the deliverable supply is divided into it, which lowers the resulting years of supply. That is why the buffer level is argued over so heavily, and why an error in it moves the headline figure more than most other inputs.
What buffer applies at the moment?
National policy has historically used a standard buffer to ensure choice and competition, with a larger buffer where the Housing Delivery Test shows under-delivery, at 5 and 20 per cent. The regime has been amended in each of the last three revisions of the Framework, so check the current levels and triggers on gov.uk and state the edition you cite.
What is the difference between the Sedgefield and Liverpool methods?
Both deal with a past shortfall. Sedgefield adds the whole shortfall to the first five years, raising the requirement and lowering years of supply. Liverpool spreads it across the remaining plan period, which has the opposite effect. Guidance has favoured dealing with shortfall in the first five years where possible.
Can an authority fail the delivery test and still have five years of supply?
Yes. The Housing Delivery Test looks backwards at three years of completions against requirement, while the supply calculation looks forward at deliverable sites. They measure different things, so an authority can fail one and satisfy the other. A submission should address both rather than assuming one follows from the other.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.