Site abnormals cost is the category that decides whether a residual land value survives contact with reality. The superstructure of a residential building is predictable within a reasonable range. Everything below ground and outside the building line is not.
That asymmetry is why a rate per square metre appraisal can be broadly right about the building and badly wrong about the scheme.
What counts as an abnormal
Anything the site imposes that a normal site would not. Deep or piled foundations because of ground conditions. Contamination requiring treatment or removal. Made ground, mine workings, or a high water table. Demolition of existing structures.
Retaining structures because of level changes. Diversion of services crossing the site. Substation provision or reinforcement of incoming capacity. Highways works required by the consent. Attenuation because of drainage constraints.
Party wall matters, rights of light settlements, temporary works because of adjoining structures, and the cost of restricted access on a constrained plot.
The list is site specific, which is precisely the point. A schedule assembled generically will miss the item that matters.
Price them separately, always
Abnormals do not scale with floor area, so they cannot live inside a rate per square metre. They scale with the site: its size, its shape, its slope, its history and its boundaries.
Each should therefore be its own line, with a value, a basis and a note stating what would confirm it. A schedule of twelve named items with values is a document a board can act on. A single allowance for abnormals is a number nobody can test.
Where an item cannot be valued at all because the information does not exist, it should be shown as unpriced with a stated reason. An unpriced known risk is more useful than a comfortable number, because it prompts either a survey or a contractual protection.
The information that resolves them
A desk study and a phase one environmental assessment cost little and eliminate a surprising number of questions. Ground investigation with boreholes costs more and resolves the foundation question, which is usually the largest single abnormal.
A topographical survey settles levels and therefore retaining structures and earthworks. Utility searches and enquiries settle diversions and capacity. A measured survey of existing structures settles demolition.
Every one of those is cheap relative to the exposure it removes, and every one is easier to commission before exchange than after. Where a vendor will not permit access, that is itself information.
Conditions and contractual protection
Where an abnormal cannot be resolved before commitment, the answer is usually contractual rather than analytical. An overage arrangement, a condition, a price adjustment mechanism or a retention against a specific item.
That converts an unknown cost into a defined commercial position. It requires the item to have been identified and described, which returns to the value of the schedule.
A land deal that treats abnormals as a lump sum contingency inside the offer price has taken the risk without pricing it, and the buyer discovers the size of that risk after they own the site.
The residual land value trap
Residual valuation is arithmetically fragile. Because land value is what remains after costs and profit are deducted from value, a modest movement in cost produces a large movement in land value.
Abnormals are the costs most likely to move and the ones least captured by a rate. A scheme appraised with a generic abnormals allowance can show a healthy land value that disappears entirely once the ground investigation returns.
Sensitivity testing on the abnormals line specifically, rather than on the build cost generally, is the useful exercise. It tells the buyer how much room the deal has for the thing most likely to go wrong.
Section agreements and obligations
Planning obligations, community infrastructure payments and highways agreements sit alongside physical abnormals and behave similarly: site specific, outside the building, and frequently underestimated at appraisal.
They also carry timing risk. An obligation that must be discharged before commencement can delay a start on site regardless of how ready the construction is, and delay has a cost of its own on a funded scheme.
Treating each as a named line with a value and a date is the same discipline applied to a different category. The residential context is set out under residential development.
What this means for you
Before you bid on a site, ask for the abnormals as a schedule rather than as an allowance. If nobody can produce one, the appraisal has not been done.
Then decide, item by item, which will be resolved by survey before commitment and which will be handled contractually. Those are the only two options; carrying them as a percentage is the third option and it is not one. The failure mode is described in hidden cost exposure.
Appraising a site and unsure about the abnormals?
Send what you have: the plan, any survey, the planning position. We will tell you what can be priced and what has to be conditioned.