Abnormal costs in a planning viability appraisal
Abnormal cost schedules are reduced for want of evidence, not because the costs are unreasonable. What to claim, what to document and how to avoid double counting.
An abnormal cost is a site-specific cost that a typical site of that type would not carry. Piling because the ground will not take strip foundations, removing 4,000 tonnes of contaminated material, diverting a high voltage main across the site, demolishing a reinforced concrete frame: those are abnormals. Standard foundations, ordinary external works and the usual statutory connections are not, however painful the invoice feels.
Planning Practice Guidance on viability expects costs to be based on evidence, and the district valuer will test whether each line is additional and properly attributable to this site. Most schedules that get reduced are reduced because the evidence was thin, not because the cost was unreasonable.
What the council will treat as normal
Build cost benchmarks such as BCIS rates already include a great deal. Arguing that something sitting inside the benchmark is abnormal is the quickest way to lose the reviewer's confidence on the rest of the schedule.
Treated as normal in almost every review:
- Standard substructure to competent ground, including a conventional trench fill solution.
- External works, drainage within the site to a normal connection point, landscaping and boundary treatment.
- Statutory service connections at standard charges.
- Professional fees within the usual percentage range for the scheme type.
- Marketing, sales and legal costs at conventional rates.
- Ordinary contingency, which is separate from and not a substitute for abnormals.
Treated as potentially abnormal, if evidenced:
- Remediation of contamination beyond routine imported topsoil.
- Piled or otherwise engineered foundations required by ground conditions.
- Demolition and asbestos removal on a previously developed site.
- Diversion or protection of existing services, including sewers, high voltage cables and gas mains.
- Retaining structures, significant level changes and ground stabilisation.
- Archaeological mitigation where an evaluation has identified deposits requiring excavation.
- Off-site highway works secured under section 278, discussed further in section 278 works or a section 106 contribution.
Evidence the district valuer expects
The reviewer at District Valuer Services is working for the council and will look for the audit trail behind each figure. A schedule of round numbers with no supporting documents will be discounted almost as a matter of routine.
What actually persuades, in descending order of weight:
- An elemental cost plan prepared by a quantity surveyor with measured quantities, not a lump sum.
- The site investigation report the engineering solution derives from, prepared to BS 5930 and, for contamination, BS 10175. A contaminated land assessment carried out by suitably qualified and experienced persons is the basis for any remediation line.
- Written budget estimates from the statutory undertaker or distribution network operator for diversions, on their own letterhead.
- Contractor quotations for demolition and enabling works, with a date and a scope.
- A remediation strategy setting out the method and volumes, rather than a rate per square metre.
- A comparison against a normal alternative, showing the difference. The abnormal is the increment, not the whole cost.
That last point catches a lot of schedules. If a piled solution costs a certain amount, the abnormal is that figure minus the cost of the conventional foundation the site would otherwise have needed. Claiming the full piling cost overstates the case and invites the reviewer to reduce every other line too.
The double counting problem
Benchmark land value in national guidance is existing use value plus a premium, and it is expected to reflect the implications of abnormal costs, site-specific infrastructure costs and professional site fees. In other words, a site with heavy abnormals should already be worth less.
The error is deducting abnormals from the appraisal while also setting a benchmark land value derived from transactions on clean sites. You cannot have both. Either the land value reflects the burden, or the appraisal does.
Two ways to handle it cleanly:
- Set the benchmark land value from comparable evidence on similarly constrained sites, and keep abnormals in the cost side, saying explicitly that the comparables carried equivalent burdens.
- Set the benchmark from clean site evidence, then adjust it downwards for the abnormals, and do not deduct them again in the cash flow.
State which approach you have taken in the first page of the appraisal. Reviewers spend a lot of time working out which one an assessment is using, and they assume the worse of the two when it is unclear.
Timing, inflation and the cash flow
Abnormals are front loaded. They usually fall in the first six to nine months, before any sales receipt, which affects finance cost as much as the headline figure. Show them in the cash flow at the point they occur rather than smearing them across the build programme.
On inflation, national guidance expects current costs and current values, so applying build cost inflation to abnormals while holding sales values flat will be challenged. If you do run a growth scenario, run it on both sides and present it as a sensitivity rather than as the base case.
Contingency deserves a word. A contingency on top of a fully evidenced abnormal is reasonable at a modest percentage. A contingency that duplicates the risk already priced into the abnormal is not, and reviewers spot it.
An illustrative example
Illustrative only: a 0.6 hectare former builders yard for 18 homes. The appraisal claims abnormals covering demolition of a concrete slab and workshop, removal of hydrocarbon contaminated made ground, piled foundations, and diversion of an 11kV cable.
The reviewer accepts the demolition and the cable diversion in full, because both are supported by quotations. The remediation line is reduced because the volume came from an assumed depth rather than from the trial pit logs. The piling line is reduced to the increment over a conventional foundation. Net effect: the abnormal schedule survives at roughly two thirds of the figure claimed, and the affordable housing offer moves accordingly.
Where reviewers and applicants usually disagree
Three arguments recur. The first is whether a cost is site specific at all, which is really an argument about the comparable evidence behind the benchmark. The second is quantity, where an assumed depth of made ground meets trial pit logs that do not support it. The third is timing, where the applicant has priced a solution the contractor has since replaced with a lower cost alternative.
None of these are resolved by narrative. They are resolved by handing over the underlying documents early, ideally at pre-application stage, so the reviewer has looked at them before the officer report is drafted. An appraisal that arrives complete, with appendices, tends to be reviewed once. One that arrives in pieces gets reviewed three times and loses momentum each round.
Presenting the schedule
Put the abnormals in a single table with four columns: the item, the quantity and rate, the evidence reference, and the normal cost being displaced. Cross refer each line to an appendix. Keep the narrative short and let the documents carry the argument.
Expect the assessment to be published. National guidance is clear that viability evidence should be publicly available, so write it knowing that objectors and the parish council will read the schedule line by line.
Where the outcome is a reduced affordable housing offer, the Affordable Housing Statement has to tell the same story as the appraisal, including on tenure, which is negotiated on its own terms as covered in affordable housing tenure split. A Viability Assessment drafted to the RICS professional statement structure gives your valuer and quantity surveyor a frame to drop their evidence into, and councils expect a suitably qualified author, so professional review before submission is strongly advised.
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.
Frequently asked questions
What makes a cost abnormal rather than normal?
It has to be site specific and additional to what a typical site of that type would carry. Standard foundations, ordinary external works, conventional drainage and normal statutory connections sit inside published build cost benchmarks and will be treated as normal. Piling forced by ground conditions, remediation, demolition and service diversions can all qualify if the evidence supports them.
Should I claim the full cost of a piled foundation as abnormal?
No. The abnormal is the increment above the conventional solution the site would otherwise have used. Claiming the whole piling cost overstates the position and gives the reviewer a reason to scrutinise every other line. Show the normal cost being displaced alongside the engineered solution, with quantities from the cost plan.
How do abnormal costs interact with benchmark land value?
National guidance expects benchmark land value to reflect the implications of abnormal costs, site-specific infrastructure costs and professional site fees. So you cannot both deduct abnormals in the appraisal and adopt a benchmark derived from unconstrained comparables. Choose one treatment, state it plainly at the front of the assessment, and apply it consistently.
Who reviews the abnormal cost schedule?
Councils commonly instruct District Valuer Services or an independent consultant to review viability evidence on their behalf. The reviewer tests whether each item is additional, attributable to this site, and supported by documents such as cost plans, ground investigation reports, contractor quotations and undertaker estimates. Round numbers without an audit trail are usually discounted.
Will the viability assessment be made public?
Generally yes. National guidance is clear that viability evidence should be publicly available so that the basis of any reduced obligation can be understood. Write the schedule on that assumption, keep commercially sensitive material to the minimum you can defend, and expect objectors and the parish council to read it closely.
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.