Whole plan viability and site specific viability

National policy expects viability to be settled at plan making, and a site specific viability assessment at application stage to be the exception rather than the routine. Whole plan viability tests whether the policies in a local plan can be delivered across the authority's likely development typologies. Site specific viability asks whether one scheme can bear those policy requirements in its particular circumstances. The second is only engaged where the applicant can show that particular circumstances justify it, and the burden sits with the applicant.

For a planning consultant that framing is the argument. Get it wrong and you are defending a viability appraisal against a policy the inspector has already found deliverable.

What whole plan viability is meant to settle

When an authority prepares a local plan it produces a viability evidence base, usually built on development typologies rather than on individual sites. The assessment tests whether a policy compliant scheme of each type can support the plan's affordable housing requirement, its infrastructure contributions, any levy charge and the policy standards on space, energy and biodiversity.

The point of the exercise is to place the burden in the right place. If the plan has been tested and found deliverable, the policy requirements form part of the plan-led system under section 38(6) of the Planning and Compulsory Purchase Act 2004, and an applicant who cannot meet them has to explain why their site differs rather than reopening the policy itself.

That is also why plan viability work is a public document. National guidance expects viability evidence at both stages to be published, and a consultant preparing an application case should read the authority's plan viability assessment before writing a word of their own appraisal. The typology closest to your site is the yardstick the officer will use.

Policies are supposed to be deliverable

An inspector examining a local plan has to be satisfied that the plan is deliverable over its period. Where the evidence shows a policy requirement cannot be met on the sites the plan relies on, the requirement is modified or the plan fails.

The practical result is a set of policy requirements that should already reflect achievable land values, build costs and returns for the plan area. National guidance on viability sets out the inputs, including gross development value, costs, land value, landowner premium and developer return, and indicates a range of return on gross development value that may be appropriate for the developer. Verify the current wording, because the guidance has been revised more than once.

The corollary is the point people forget. The price actually paid for the land is not a justification for failing to meet policy. A purchase made on the assumption of a lower affordable housing contribution is a commercial risk taken by the purchaser, and an Affordable Housing Statement that relies on the acquisition price is asking the wrong question.

When a site specific assessment is justified

Guidance contemplates site specific assessment in defined situations rather than as a general right, and the strongest cases share a common feature. Something about the site is materially different from the typology the plan tested.

The recurring examples are worth listing:

• Abnormal costs that are demonstrable and site derived, such as remediation of contamination, ground stabilisation, flood mitigation works or the diversion of services.
• A site or building subject to constraints the typology did not model, for example a listed structure requiring conservation-grade repair.
• Infrastructure requirements disproportionate to the scheme's scale, such as an off-site highway scheme or a new access over third party land.
• A development type the plan did not test at all, which is common for specialist housing, purpose-built student accommodation and unusual mixed use schemes.
• Material change in market or cost circumstances since the plan was examined.

Each of those has to be evidenced rather than asserted. Abnormal costs need a cost consultant's build-up tied to a ground investigation or a structural survey, not a contingency allowance described as abnormal.

Changed circumstances since adoption

The changed circumstances argument is the most common and the most often mishandled. It works where the change is real, documented and material, and it fails where it is a general assertion about market conditions.

What lands with a decision maker is evidence of the shift and evidence that it affects this site. Published cost indices for build cost inflation. Comparable transactions for values in the locality. A specific regulatory change that adds cost after the plan was tested, such as a new building regulations uplift or a new statutory requirement. The age of the plan viability evidence itself, which on an older plan may predate several changes.

What does not land is the argument that the plan is old and therefore its policies are unviable. Inspectors look for the mechanism, the number and the causal link to the appraisal.

There is a second-order point here that is easy to miss. Where an authority accepts a reduced contribution on changed circumstances grounds, it will frequently ask for a review mechanism so that improved values are captured later. Overage clauses and review mechanisms in a section 106 are the usual price of the argument succeeding.

Evidence expected at each stage

Benchmark land value is where most disagreement concentrates. Guidance directs assessment by reference to existing use value plus a premium to the landowner, informed by the policy requirements of the plan. A Viability Assessment that starts from a market purchase price and works backwards will be rejected, and rightly so.

How inspectors treat the distinction

Appeal decisions and plan examination reports read differently on viability, and the distinction is instructive.

At examination, the inspector is asking whether the plan as a whole works, and individual site evidence carries limited weight. At appeal, the inspector takes the adopted policy as the starting point, then considers whether the applicant has shown that particular circumstances justify departure. The weight given to a site specific appraisal is a matter for the decision maker, which in practice means a well-evidenced appraisal from a qualified valuer, audited by the authority's own adviser, carries weight and a thin one does not.

Two habits help. Agree as many inputs as possible with the authority's adviser before the decision, narrowing the dispute to one or two figures. And be straightforward about the appraisal's sensitivities, because an appraisal that only works at one set of assumptions invites the question of what happens if values improve.

Framing the argument at application stage

Set the case out in this order, because it mirrors how the officer will assess it.

1. Identify the adopted policy requirement and the plan viability typology closest to the scheme.
2. State plainly what is different about this site, in one paragraph, before any arithmetic.
3. Evidence each difference with a third party document, whether cost plan, survey or transaction data.
4. Present the appraisal with benchmark land value derived from existing use value plus premium.
5. Offer a review mechanism, since resisting one usually costs more than accepting it.
6. Say what the scheme can deliver, not only what it cannot.

Where the scheme is small, check the national threshold position first. Affordable housing on small sites may mean the requirement does not apply at all, which is a faster answer than a viability argument. And where existing floorspace is being replaced, the vacant building credit may reduce the requirement without any appraisal.

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. For specialist technical reports, qualified professional input may be required, and a viability appraisal should be prepared by a suitably qualified valuer.