Using build cost indices in a planning appraisal
An index figure without a base date tells a reviewer nothing. Here is how to move a build cost to the valuation date, forecast forward without double counting, and document it so it gets agreed.
A construction cost index is only useful in a planning appraisal if the base date is stated. An index figure without a base date tells a reviewer nothing, because the whole purpose of indexing is to move a cost from the date it was estimated to the date it will be spent. Most disputes about build cost inflation in viability work are really disputes about which date the cost estimate belongs to.
Get the base date right and the rate becomes a secondary argument. Get it wrong and no amount of sensitivity testing rescues the figure.
Why the base date matters more than the rate
Every build cost in an appraisal carries an implicit date. A cost per square metre taken from a published tender price source is a figure at a particular quarter. A quantity surveyor's estimate is priced at the date of the estimate. A contractor's quotation is live for a stated period and then it is not.
The appraisal then has to do two separate things with that number. It has to bring the cost forward to the valuation date, which is a question of historic index movement and is verifiable. And it has to project the cost forward across the build programme, which is a forecast and is not.
Mixing those two steps is where double counting starts. An appraisal that takes a cost already inflated to the valuation date and then applies the full historic movement again has overstated cost by whatever the index moved in between. Reviewers look for exactly this, so label every cost line with its base date and the adjustment applied.
Which indices are used and for what
Different indices measure different things, and substituting one for another changes the answer.
| Measure | What it tracks | Typical use in an appraisal |
|---|---|---|
| Tender price indices | What contractors are actually bidding, including margin and market conditions | Moving a historic cost estimate to the valuation date |
| Building cost indices | Input costs of labour and materials, excluding market margin | Checking whether input cost pressure explains a movement |
| Output price indices published by the Office for National Statistics | Prices of construction output by sector | Cross-checking a tender based assumption against national data |
| Material price indices | Specific commodity movements | Explaining a particular element, such as structural steel or glazing |
Tender price movement and building cost movement diverge, sometimes sharply, because contractor margin expands and contracts with workload. An appraisal that uses an input cost index to justify a tender price increase is comparing the wrong things, and a reviewing surveyor will say so.
Where a scheme has a real contractor quotation for a defined scope, that evidence outranks any index. Indexation is a substitute for evidence, used when the evidence is dated.
Separating inflation from contingency
Contingency covers the unknown within the defined scope. Inflation covers the movement in price of the known scope over time. They are different allowances and they should sit on different lines.
The common failure is a single combined allowance, often expressed as a percentage, that is then defended as both. A reviewer will test it by asking what the contingency covers if inflation is already allowed, and the answer is usually silence.
Set it out explicitly:
- Base construction cost at a stated base date, with the source.
- Indexation from the base date to the valuation date, with the index used and the movement shown.
- Forecast build cost inflation across the programme, with the assumed annual rate and the programme length.
- Contingency on the base cost, at a percentage justified by the design stage reached.
- Abnormal costs itemised separately, because they are not a percentage of anything.
Note that contingency percentages fall as design information improves. A 10 per cent allowance at concept stage and the same 10 per cent on a scheme with a full technical design and a tendered price are not both defensible.
Sensitivity testing an inflation assumption
The appraisal should show how the residual land value responds to the inflation assumption, because that is what tells the decision maker how much weight the figure carries.
Run the test on the forecast element only. Hold the base cost and the historic indexation fixed, because those are evidenced, and vary the forward rate across a range that spans plausible outcomes. Present the output as a short table of residual value against rate, and state the point at which the scheme fails to support the policy contribution.
Two refinements make the test credible. Vary sales values in the same test, because build cost inflation and house price inflation are not independent, and an appraisal that inflates costs while holding values flat across a three year programme is not a neutral assumption. And test the programme length as well as the rate, since a longer build exposes more of the cost to inflation.
Where the appraisal supports a reduced affordable housing offer, the sensitivity output is the part the authority's reviewer reads first. Budgeting a viability assessment and its review mechanism covers how the review mechanism then handles outturn against assumption.
Where reviewers challenge the figure
Authority reviewers and their consultants return to the same points, so anticipate them in the text rather than in correspondence.
They challenge the base date, asking which quarter the cost belongs to and whether the adjustment to the valuation date has been applied once or twice. They challenge the index selected, asking why a tender price measure was used where input costs would be the fairer comparison, or the reverse. They challenge the forward rate against published forecasts, and an unsupported rate above the general run of published forecasts is difficult to hold. They challenge contingency stacking, where a percentage sits on top of a cost that already includes a risk allowance. And they challenge abnormals, asking for the engineering evidence behind each one rather than a lump sum.
The honest difficulty is that forward inflation is a forecast and nobody can evidence it. The way to deal with that is to say so, give the source for the rate used, and show the sensitivity. An appraisal that presents a forecast as a certainty invites the reviewer to substitute their own.
Updating an appraisal mid determination
Determination periods on major schemes run for months, and a viability appraisal prepared at submission can be out of date by the time the section 106 is drafted.
Agree the approach with the authority early. Two workable options. Fix the valuation date at submission and deal with movement through the review mechanism, which keeps the negotiation stable. Or update the appraisal at an agreed point, with both sides refreshing cost and value inputs on the same date and the same index.
What does not work is a one sided update. An applicant who refreshes costs upwards while leaving sales values at the submission date will lose the argument and some credibility with it. If the appraisal is updated, update both sides of it.
Documenting the source
Each cost line needs four pieces of information, and an appraisal that provides them is much faster to agree.
- The figure and the unit, with the measurement basis stated.
- The source, named specifically, with the publication and the edition or quarter.
- The base date of that figure.
- The adjustment applied to reach the valuation date, shown as a calculation rather than a result.
Attach the quantity surveyor's cost plan as an appendix rather than summarising it, and keep the appraisal spreadsheet's input tab separate from its calculation tab so a reviewer can change one assumption without rebuilding the model. Other cost lines in the same appraisal deserve the same treatment, including the items covered in budgeting utility connections and diversions, which are routinely understated at submission stage.
Viability work needs a suitably qualified author, normally a chartered surveyor working to the recognised professional guidance, and professional review is strongly advised before the appraisal is submitted. 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. See PlanScope pricing for the per-report and Professional options.
Frequently asked questions
Why does the base date of a build cost matter so much?
Because indexation exists to move a cost from the date it was priced to the date it will be spent. Without a stated base date a reviewer cannot tell whether the adjustment to the valuation date has been applied once, twice or not at all. Double counting between the historic adjustment and the forward forecast is the commonest error in viability cost lines.
What is the difference between a tender price index and a building cost index?
A tender price index tracks what contractors are actually bidding, which includes margin and market conditions. A building cost index tracks input costs of labour and materials without margin. The two diverge as contractor workload changes, so using an input cost measure to justify a tender price movement compares different things and invites a challenge.
Should inflation and contingency be separate lines?
Yes. Contingency covers the unknown within a defined scope and should fall as design information improves. Inflation covers movement in the price of the known scope over time. A single combined percentage cannot be defended as both, and a reviewer will ask what the contingency covers once inflation has been allowed separately.
How should an inflation assumption be sensitivity tested?
Hold the evidenced elements fixed, the base cost and the historic indexation, and vary the forward rate across a plausible range. Show residual land value against rate and state where the scheme stops supporting the policy contribution. Vary sales values in the same test, because inflating costs while holding values flat is not a neutral assumption.
Can an appraisal be updated during determination?
Yes, and on long major applications it often has to be. Either fix the valuation date at submission and handle movement through the review mechanism, or update at an agreed point with both sides refreshing cost and value inputs on the same date. A one sided update that raises costs while leaving values unchanged will not be accepted.
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.