Residual valuation against comparable land transactions

Residual valuation vs comparables is the argument that decides most contested viability cases, and the two methods disagree for a structural reason rather than because one side has done the arithmetic wrong. A residual appraisal works backwards from the finished scheme: take the gross development value, deduct build costs, fees, finance and developer profit, and what remains is what the land can bear. Comparable evidence works from what similar land actually sold for. The gap between the answers is usually policy, because prices paid in the market embed assumptions about how much affordable housing the buyer expects to provide.

National guidance resolves that tension in a particular direction, and knowing which direction saves a lot of argument.

Two routes to a land value

Benchmark land value is the figure both methods are trying to establish. National planning guidance sets it as existing use value plus a premium sufficient to persuade a reasonable landowner to sell, and it is explicit that the premium should reflect policy requirements rather than hope value. That framing matters because it rules out the simplest version of the comparables argument.

What the residual method actually assumes

A residual appraisal is a long division sum dressed as a model. Its output is the difference between two large numbers, so a small percentage change in either produces a large percentage change in the residual.

Move sales values by 5 per cent on a scheme where land is 15 per cent of gross development value and the residual moves by a third. That sensitivity is the method's weakness and its usefulness. It is why a viability case should always be presented with sensitivity testing rather than as a single figure, and why a case officer who only ever sees one scenario should ask for more.

The inputs that carry the argument, in rough order of how often they are contested: sales values and the evidence behind them, build costs and whether they come from a measured estimate or a cost index, developer profit, finance rate and period, and abnormal costs. National guidance refers to 15 to 20 per cent of gross development value as a suitable return for market housing, with a lower figure for the affordable element, and a scheme claiming more than that range without explanation will be challenged.

Abnormals are where good faith is tested. A ground condition cost supported by a site investigation report is evidence. The same figure described as an allowance is an assertion.

Why comparable evidence is hard to adjust

Land transactions are not houses. There are few of them, they are rarely identical, and the terms are often confidential or structured in ways the headline price does not reveal.

Four adjustments are needed on almost every comparable, and each one is contestable. The date, which needs a market movement adjustment over a period when the market may not have moved uniformly. The planning status, since land with permission, land with an allocation and land with neither are different commodities. The tenure and mix assumed by the buyer. And the deal structure, because an overage arrangement, a deferred payment or a conditional contract changes what the headline figure means.

Even with all four handled honestly, you end up with a range rather than a figure. Present the range.

Policy compliance embedded in the price

Here is the structural reason the two methods diverge. If developers in an area have consistently bought land assuming they would negotiate affordable housing down from 35 per cent to 20 per cent, prices paid reflect that expectation. Use those prices as the benchmark and you import the under-delivery into the benchmark itself, then use the benchmark to justify repeating it.

National guidance addresses this directly, requiring that where market evidence is used it should reflect policy compliant schemes, and that a transaction involving a scheme which did not meet policy requirements is not a sound comparison. In practice that reduces the usable sample considerably, sometimes to nothing in an area where few recent schemes were policy compliant.

The answer is not to abandon comparables. It is to use them as a check on the residual figure rather than as the benchmark itself, and to say in the report which transactions were excluded and why.

Using both as a cross-check

The sensible appraisal runs both and explains the relationship between them.

1. Establish existing use value from the site's current lawful use, evidenced.
2. Derive the benchmark land value as existing use value plus a premium, with the premium reasoned rather than assumed.
3. Run the residual appraisal on the policy compliant scheme and see whether it clears the benchmark.
4. Test the benchmark against transaction evidence, with each comparable adjusted and each adjustment stated.
5. Where the two diverge, say why, and run sensitivities on the inputs driving the divergence.

A divergence is information rather than an error. If the residual on a policy compliant scheme falls well below the transaction evidence, the likely explanations are that the market has been paying for policy non-compliance, that the scheme design is inefficient, or that the cost inputs are high. Each of those can be tested. If the residual sits well above the comparables, the usual explanations are optimistic sales values or a build cost that has not caught up with the measured position.

Presenting it so it survives scrutiny

Viability assessments are expected to be made public, and an appraisal written for the client's eyes alone reads badly when a residents' group has it.

Three habits make the difference. State every assumption with its source on the face of the appraisal rather than in an appendix. Show the sensitivity table, including the scenarios that are uncomfortable. And separate the facts from the judgements, so that a reviewer can accept your evidence while disagreeing with your premium.

The report also has to connect to the planning case. An Affordable Housing Statement setting out what is offered and how it is secured should reconcile line by line with the appraisal, and a Viability Assessment that contradicts it hands the objectors an easy point. Where the scheme may improve, a review mechanism is the standard response, and our note on affordable housing review mechanisms sets out how they are drafted.

Viability work is a report type where councils expect a suitably qualified author, usually a chartered surveyor working to the professional standards for financial viability in planning, and professional review is strongly advised. An AI-assisted draft can carry the policy narrative and the structure, and it can flag where inputs are missing. The valuation judgement is the valuer's.

If an authority rejects the appraisal, the argument reopens at appeal, which is covered in our piece on when an inspector reopens viability.

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.