Abnormal cost evidence a council's reviewer will accept
Abnormal costs are where most viability appraisals lose credibility. Every line needs a technical report behind it, a quantity, a dated rate and a source, or the reviewer removes it.
Abnormal costs viability evidence works line by line, and each line needs four things: a description of the physical problem, the technical report that identified it, a measured quantity, and a rate with a named source and a date. A schedule that reaches £1.8 million on eleven lines with no cross-references will be reduced to whatever the reviewer can verify, which is usually the remediation figure and nothing else. That is not the reviewer being difficult. Planning Practice Guidance expects cost assumptions to be supported by evidence, and abnormals are the part of the cost plan that is least susceptible to a benchmark check.
The reviewer's starting position is that the base build rate already includes normal construction. Anything claimed above it has to be shown to be additional in fact.
What counts as an abnormal
An abnormal is a cost arising from a site-specific condition that a standard build rate does not cover. The categories that survive scrutiny are narrow.
- Ground remediation to make the site suitable for the proposed use, including gas protection measures, capping and the disposal of contaminated material at the correct waste classification.
- Foundation design beyond what the base rate assumes, where made ground, mining legacy, shrinkable clay with adjacent trees or a high water table require piling.
- Demolition and structural works to retained fabric, including facade retention and temporary works.
- Diversion or abandonment of statutory services crossing the site, and off-site reinforcement of a network that cannot take the load.
- Flood resilience or level changes driven by a finished floor level requirement.
- Highway works under a section 278 agreement, where not otherwise counted as an obligation.
- Archaeological mitigation where a written scheme of investigation has been required.
- Asbestos removal identified by survey.
What does not count is longer than what does. Standard strip or raft foundations. Site clearance and levelling of a normal kind. Statutory connections at published rates. Professional fees, which belong in their own line. Marketing, finance and profit. And the developer's own allowance for things going wrong, which is contingency.
Tracing each item to a technical report
The rule that saves most arguments is simple: no abnormal without a report. A piling line needs a ground investigation with borehole logs and a foundation options appraisal. A remediation line needs a Phase 2 investigation, a remediation strategy and a waste classification. An archaeology line needs an evaluation and a written scheme of investigation, not a desk-based assessment that says the potential is moderate.
Where the report is not yet available, say so and mark the line as provisional with the basis of the allowance stated. A reviewer will often accept a provisional sum supported by a named assumption. They will not accept a firm figure with no report, because they have no way to test it.
Sequence matters for cost as well as for credibility. A Geotechnical Assessment that postdates the appraisal produces a foundation cost nobody can reconcile with the drawings. Commission the investigation before the appraisal, not alongside the section 106 negotiation, and keep the contaminated land assessment findings and the remediation cost in the same version control.
Cost sources and who prepared them
State the provenance of every rate.
| Cost type | Acceptable source |
|---|---|
| Base build | Published cost index data for the building type and location, adjusted and dated |
| Abnormal construction | Quantity surveyor's measured cost plan, with quantities |
| Remediation | Specialist contractor quotation or remediation strategy cost estimate |
| Waste disposal | Gate fees by waste classification, with the classification stated |
| Services diversion | Statutory undertaker's budget estimate or formal quotation |
| Highway works | Section 278 estimate from the highway authority or a civils quotation |
| Archaeology | Contracting unit quotation against the written scheme of investigation |
Two provenance problems recur. A cost plan prepared for the contractor's tender, which includes preliminaries and overheads already in the base rate. And a quotation that is two or three years old, uplifted by an inflation figure the appraisal does not otherwise apply. Date every rate, and apply inflation consistently across the whole appraisal or not at all.
Items commonly struck out
Reviewers remove the same lines with fair regularity, and knowing the list lets you either drop them or evidence them properly.
- Piling described as abnormal where the ground investigation shows conventional foundations are feasible and piling was a programme choice.
- Remediation to a standard beyond what the proposed end use requires, for example residential standards on a commercial parcel.
- Contingency plus a separate risk allowance on the same items.
- Site-wide preliminaries counted in the base rate and again in the abnormals schedule.
- Section 278 works that also appear in the section 106 obligations, so the same money is deducted twice.
- Abnormal professional fees where a percentage fee has already been applied to the total construction cost including the abnormals.
- Costs of overcoming a constraint the applicant created, such as a layout that puts a block over a live sewer easement.
- Demolition of buildings whose removal was assumed in the benchmark land value.
The last one is worth dwelling on. If the benchmark land value was derived on the basis of a cleared site, the clearance cost sits in the land value and cannot be claimed again as an abnormal. That single inconsistency accounts for a meaningful share of disputes, and it is the first thing a reviewer with a valuation background checks.
Double counting against contingency
Contingency covers the unknown. An abnormal is a known cost with a measured quantity. Where both appear on the same item, one has to go.
The workable convention is to apply contingency to the base build and to the provisional abnormals at a stated rate, and to exclude firmly quoted abnormals from the contingency base. Say which convention you have used in a note under the schedule. A reviewer who can see the convention will argue about the percentage. A reviewer who cannot will assume the worst and strip the contingency out.
Risk and contingency are also where optimism enters through the profit line. A scheme claiming both a high profit margin for risk and a full contingency on quoted abnormals is claiming for the same risk twice, and the Planning Practice Guidance approach to profit expects the margin to reflect the actual risk of the scheme.
Presenting the schedule for review
Give the reviewer a single table they can audit without opening five appendices, with a column for the source document and its date. Number the lines and use the same numbering in the technical appendices.
Include a short covering note that states the base build source and date, the treatment of preliminaries, the contingency convention, the inflation position, and which lines are provisional. Four paragraphs. It saves a fortnight.
Where the appraisal is confidential, expect to publish the schedule in redacted form anyway, because the authority has to be able to explain its decision. The practical limits of confidentiality are set out in the note on viability at appeal and confidential appraisals, and the wider structure of the submission sits in the Viability Assessment.
Responding when the reviewer disagrees
Ask for a without prejudice meeting between the two quantity surveyors before any formal exchange. Most disagreements are about scope rather than rate, and scope is resolved faster in conversation.
Then produce a joint statement listing agreed items, disputed items with each side's figure and the reason, and the effect of each dispute on the residual. A committee can make a decision on that document. It cannot make a decision on two appraisals that differ by £2 million with no explanation of where.
If a line cannot be agreed, offer a review mechanism rather than a fight. An early stage review or a late stage review in the section 106 agreement lets the authority test the outturn and recover more obligation if the cost did not materialise, which frequently produces a resolution that argument does not.
See PlanScope pricing if you want a structured first draft of the supporting statements while the cost work is being finalised.
This is general guidance, not legal advice. For specialist technical reports, qualified professional input may be required. 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 an abnormal cost in a viability appraisal?
A cost arising from a site-specific condition that a standard build rate does not cover, such as remediation, piling driven by ground conditions, facade retention, service diversions or archaeological mitigation. It has to be additional to the base build rate, measured, and supported by the technical report that identified the condition in the first place.
Why did the reviewer remove our piling costs?
Usually because the ground investigation did not show that conventional foundations were unfeasible, so piling reads as a programme or buildability choice rather than a site constraint. Provide the borehole logs and a foundation options appraisal comparing the alternatives, with the reason conventional foundations were discounted stated by the engineer.
Can contingency and abnormals both be claimed on the same item?
No. Contingency covers unknown costs and an abnormal is a known cost with a measured quantity. The workable approach is to apply contingency to the base build and to provisional abnormals only, excluding firmly quoted lines, and to state that convention in a note beneath the schedule so the reviewer can see it.
Do demolition costs count as an abnormal?
Only if the benchmark land value was derived on the basis of a site with the buildings still standing. Where the land value assumed a cleared site, the clearance cost is already reflected in the land price and claiming it again double counts. Check the valuation basis before putting demolition in the schedule.
How should abnormal costs be presented?
As one numbered table with a description, the quantity, the rate, the total, the source document and its date, cross-referenced to the technical appendices by the same numbers. Add a covering note stating the base build source, the treatment of preliminaries, the contingency convention, the inflation position and which lines remain provisional.
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.