A viability appraisal input schedule a reviewer can test

A viability appraisal is only as persuasive as its input schedule. The arithmetic is standard and any competent reviewer can rebuild it in an afternoon, so arguments are almost never about the model. They are about the twelve or fifteen numbers fed into it, where each came from, and what date it was fixed at. Setting out a viability appraisal inputs schedule as a standalone document, separate from the residual calculation, is the single change that most improves how an assessment is received.

Planning Practice Guidance on viability expects assessments to be transparent and publicly available, with the inputs and assumptions set out so that others can test them. The RICS professional guidance on assessing viability in planning carries the same expectation. An appraisal where the inputs are buried in the printout of a cashflow does not meet it.

Separating inputs from calculations in the model

Build the appraisal in three layers. An input schedule, a calculation engine and an output summary. The input schedule holds every assumption with its evidence. The calculation references the schedule and contains no hard-coded numbers. The output reports the residual land value and the surplus or deficit against the benchmark.

That structure has a practical payoff during negotiation. When the council's reviewer disputes the sales value, you change one cell and reissue, instead of arguing about whether the revised figure has been applied consistently. It also makes the sensitivity work honest, because the ranges attach to named inputs rather than to a conclusion.

Each row of the schedule carries five fields: the input, the value, the unit, the source of evidence, and the date the evidence relates to. Add a sixth for the sensitivity range you are prepared to test.

Gross development value and the evidence for it

Market housing values are the input that moves the answer most, so they take the most evidence. State them as a rate per square metre of net saleable area by unit type, not as a single blended figure, and support them with comparable transactions identified by address, date, floor area and achieved price. New build comparables carry more weight than second-hand sales, and anything older than six months needs an explanation.

Affordable housing transfer values belong in the schedule as a separate line for each tenure, expressed as a percentage of open market value or as a capitalised rent. Where a registered provider has quoted, say so and give the date. Where no quote exists, state the assumption and the basis for it.

For commercial floorspace, give the rent per square metre, the yield, the void period and the rent-free assumption separately. Collapsing those into a single capital value hides the inputs a reviewer most wants to test.

Build costs, base dates and abnormals

Base construction costs normally come from a published cost index for the building type, adjusted for location and brought to a stated base date, or from a cost plan prepared by a quantity surveyor. Either is acceptable. What is not acceptable is a rate with no index, no location factor and no date.

Keep abnormals on separate lines with their own evidence. Demolition, piling, remediation, retaining structures, diversions, archaeology, off-site highway works and any s278 agreement each deserve a row traceable to a drawing, a specification or a quotation. Lumping £1,200,000 of abnormals into one line invites the reviewer to discount the lot.

Two details that get missed. Say whether the cost rate includes external works and site infrastructure or whether those sit on separate lines, because double counting and omission are equally common. And state the contingency as a percentage of base cost with the percentage visible, rather than folding it into the rate.

Professional fees, finance and developer return

Professional fees are usually a percentage of construction cost. Give the percentage, say what it covers and note anything excluded and priced separately, such as planning fees, surveys or the levy.

Finance needs four inputs: the interest rate, the arrangement or exit fees, the proportion of cost assumed to be debt funded, and whether interest is calculated on a cashflow or on a straight-line basis across the build period. The last of those changes the answer materially and is often left unstated.

Developer return is the input most often treated as a fixed truth. It is a judgement. The viability guidance indicates that a return of 15 to 20 per cent of gross development value may be a suitable assumption for plan making, with a lower figure potentially appropriate for affordable housing, and it asks for the basis of the figure used to be set out. State the metric as well as the number, because a profit measured on cost and a profit measured on value are different animals, and give the programme assumptions that drive the finance cost.

Benchmark land value and the approach taken

This is where most disputes end up. The guidance directs benchmark land value to be based on existing use value plus a premium to the landowner, with the premium justified by reference to market evidence and set at the minimum return at which a reasonable landowner would release the site.

The schedule should therefore show, as separate rows, the existing use value with its evidence, the premium as a figure and a percentage, and the resulting benchmark. Where an alternative use value is argued, state the alternative use, whether it has permission or a realistic prospect of it, and the value basis.

Avoid presenting the price paid as the benchmark. The guidance is explicit that the price paid for a site is not a justification for failing to meet policy requirements. Including it as context is fine. Relying on it is not.

Policy contributions and the order they are applied

List every policy requirement as an input, costed, with the policy reference beside it:

Apply them in the order the plan sets out, and say which requirement you are proposing to reduce and why. An appraisal showing a deficit without identifying which contribution is being traded is not a proposal, it is a complaint. Where affordable housing is the item in question, the reasoning belongs in the affordable housing statement as well as in the appraisal.

Sensitivity ranges for each contested input

Run sensitivities on the inputs that are actually contested rather than on all of them. Sales values, build cost, developer return and benchmark land value are the usual four. Present the result as a table showing the surplus or deficit at the low, central and high case for each, and say which combination would restore policy compliance.

Where a review mechanism is on the table, the sensitivity work is what justifies the trigger points. An early-stage review set against a sales value the appraisal itself calls uncertain tends to be accepted more readily than a flat refusal to accept any review.

Presenting the schedule so a reviewer can test it

Put the input schedule at the front of the viability assessment, before the narrative. Number every row so the text and the appraisal printout can cite it. Append the comparable evidence, the cost plan and the registered provider correspondence as separate appendices rather than embedding extracts.

One illustrative example of the discipline. On a notional scheme of 48 homes, an appraisal that states "build cost £2,150 per m² of gross internal area, from a published cost index for estate housing, location factor applied, rebased to the first quarter of the current year, excluding external works and abnormals which are at rows 14 to 19" can be checked in minutes. The same appraisal saying "build cost £2,150 per m²" cannot, and the reviewer will substitute a figure of their own. The numbers here are illustrative only.

A reviewer who can test your schedule will argue about two or three inputs. A reviewer who cannot will argue about all of them. Before submission, run the appraisal past the same internal quality check you would apply to any submission, and browse the report types to confirm which supporting statements the scheme needs alongside it.

Viability work is specialist. PlanScope can produce an AI-assisted draft structured to the policy tests, and councils normally expect appraisals prepared or reviewed by a suitably qualified valuer. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice.