Abnormal costs in a viability assessment: what counts

Abnormal costs are the site-specific items a scheme has to pay for that a development on a clean, level, serviced site would not. Piling because the ground will not take strip foundations. Remediating made ground. Diverting a sewer, a gas main or an 11kV cable. In a viability assessment the abnormal costs line usually carries the largest gap between the applicant's appraisal and the council's own review, because it is the easiest place to move a scheme from viable to unviable without changing anything on the drawings.

Reviewers know that. So the abnormal schedule gets read line by line, and anything that looks like a normal build cost wearing a different hat comes straight back out.

What counts as an abnormal cost

The working definition most reviewers apply is a cost that falls outside the build cost rate already being used. If the appraisal takes a BCIS rate for a three-storey block of flats, that rate already contains a normal substructure, normal drainage, normal external works and normal preliminaries. An abnormal is what the site demands over and above that baseline because of its own ground conditions, its history, its levels, its services or its access.

Items that usually qualify: piled or raft foundations where standard footings would otherwise do, demolition of existing structures, removal of obstructions and old slabs, contamination remediation and validation, gas protection membranes, retaining structures to deal with a level change, diversion or reinforcement of statutory undertakers' apparatus, off-site highway works secured under section 278, archaeological mitigation, and abnormal drainage such as a pumped connection where gravity is not available.

Items that usually do not: the cost of the design itself, standard external works, normal preliminaries, sales and marketing, anything already priced inside the build rate, and anything the developer chose rather than the site required. Enhanced specification is a choice. It belongs in the base cost, and a reviewer will move it there.

Evidence a council will accept

An abnormal cost is only as good as the document behind it. A schedule of round numbers with no supporting paper is the single most common reason for a reduction, and on a contested application it is the fastest way to lose credibility on every other input as well.

What holds up:

• A cost plan or elemental estimate from a quantity surveyor, dated, with the abnormals identified separately from the base build cost.
• Contractor quotations for the specific work, ideally more than one, on the specific site.
• The technical report that creates the need: a ground investigation for piling, a Phase 2 report for remediation, a topographical survey for retaining structures, a sewer record and a diversion budget estimate from the undertaker.
• A drawing that ties the cost to a location on the site, so the reviewer can see that the piled area and the built area are the same area.

What does not hold up: an allowance carried forward from a different site, a rate per square metre with no basis, a figure described as a provisional sum with no scope attached, or a quotation for work at a scale the application does not propose. If a contaminated land assessment has not been done, a remediation figure is an estimate of an estimate, and it should be presented as one rather than dressed up as a price.

Remediation, demolition and services diversion

These three account for most of the money on brownfield sites, and each has a characteristic failure mode.

Remediation costs get challenged where the strategy is more expensive than the risk requires. A dig-and-dump figure for material that could be treated on site, or a blanket cap across a site where hotspots are localised, will be questioned by any reviewer who has read the Phase 2 report. Tie the cost to the remediation strategy, and tie the strategy to the conceptual site model and the end use. Residential end use drives a more cautious position than commercial, and saying so in the text is better than leaving the reviewer to infer it.

Demolition is usually accepted where the structures exist and the photographs show them, but asbestos removal needs a refurbishment and demolition survey behind it. A pre-redevelopment audit can also set out what is being recovered and reused, which matters where the council has a circular economy policy.

Services diversions are the slowest to evidence because the undertaker controls the timetable. Budget estimates are available but they expire, and they are not firm quotations. Say which they are. A reviewer who spots an expired budget estimate presented as a fixed price will discount it, and reasonably so.

Double counting against the land value

This is where most abnormal cost arguments are actually won or lost, and it is the part applicants most often get wrong.

National viability guidance expects the benchmark land value to be established using the existing use value plus a premium approach. If the site is a former industrial yard with contamination and obstructions, that condition is already reflected in what the land is worth. Claiming the full abnormal cost as a deduction, while also adopting a benchmark land value drawn from clean comparable sites, counts the same disadvantage twice.

The test to apply before submitting: does the benchmark land value assume a site in its present condition, or a site already cleared and remediated? If the comparables are clean sites, the abnormals have to be netted against the land value rather than stacked on top of the build cost. Reviewers are alert to this. Set out the reasoning explicitly, because an assessment that addresses the point is treated very differently from one that hopes nobody notices.

Contingency and how it is treated

Contingency and abnormals are related but they are not the same thing, and applying a percentage contingency to the whole cost including abnormals will usually be resisted.

The logic is that an abnormal cost supported by a quotation carries less risk than a base build cost carried at a published rate, so it needs less contingency, not more. Where uncertainty is genuine, for instance remediation quantities before intrusive investigation, the honest position is a higher allowance on that item with the reason stated, rather than a blanket uplift across the appraisal. An illustrative case: a scheme carrying £900,000 of abnormals, of which £600,000 is a firm piling quotation and £300,000 is a pre-investigation remediation allowance, is better presented with a modest contingency on the piling and a larger one on the remediation, each explained. That is illustrative only, but the structure is what reviewers look for.

Independent review of the figures

Most authorities commission an independent viability review, paid for by the applicant, and the reviewer will normally be a chartered surveyor working to RICS professional standards on financial viability in planning. Expect the review to interrogate the build cost source and date, the abnormal schedule and its evidence, the benchmark land value, the sales values, the profit margin and the finance assumptions.

Two practical points. First, the review is usually published, subject to limited redaction, because national guidance expects viability evidence to be transparent. Write the assessment knowing that objectors will read it. Second, the reviewer will ask for the underlying documents rather than accept the summary, so assemble the ground investigation, the cost plan and the quotations before submission rather than after the first challenge.

And if an item cannot be evidenced yet, a deferred mechanism such as a review at an agreed trigger point is often more persuasive than an unsupported number. It keeps the application moving.

Where abnormals get rejected

The recurring reasons are consistent enough to work as a pre-submission checklist. Costs already inside the build rate. Areas that do not reconcile with the layout. Quotations for a different scheme or a different site. Optimism bias in the remediation strategy. Double counting against a clean-site land value. Contingency applied twice. A schedule with no dates, so nobody can tell whether the prices are current.

Viability is one of the few planning arguments decided almost entirely on documents rather than judgement about design, so the paperwork is the case. A viability assessment drafted with the abnormal schedule cross-referenced to the reports that justify each line will survive review in a way that a spreadsheet extract never does.

PlanScope can draft the narrative structure of a viability assessment, with the abnormal cost section set out for your quantity surveyor's figures to drop into, and flag where site evidence is missing. Enter your postcode to see which reports your site needs, then send the draft for professional review before submission.

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 viability evidence should be prepared or checked by a suitably qualified surveyor.