Canterbury & Kent
Diminution in Value Calculations and Expert Witness Reports
How diminution in value is calculated for property damage, nuisance and right-to-light claims, and how an RICS valuer's expert witness report supports it.
Diminution in value is the reduction in a property’s worth caused by a specific event or condition — damage, a defect, nuisance, or the loss of light from a neighbouring development. In many property disputes it is the figure that decides the size of a claim, and it needs to be assessed by a valuer whose evidence will stand up to scrutiny.
What is diminution in value?
Put simply, it is the difference between what the property would be worth without the problem and what it is worth with it. That gap is the loss. It comes up in claims for building defects and damage, Japanese knotweed, subsidence, right-to-light and rights-of-light disputes, boundary and party wall matters, and professional negligence.
How is diminution in value calculated?
A valuer establishes two figures on the same valuation date. The first is the property’s open market value in its assumed sound or unaffected state. The second is its value in the actual affected state, taking account of the cost and disruption of putting things right and any lasting stigma that remains even after repair. The diminution is the difference. The assessment is carried out on Red Book principles by a valuer on the RICS Register of Valuers, using comparable evidence to support each figure.
The role of the expert witness report
Where a claim may go to court, the valuation is presented as an expert witness report that complies with Part 35 of the Civil Procedure Rules. That means the surveyor’s overriding duty is to the court, not to the party paying the fee, and the report sets out the facts, the reasoning and the valuer’s independent opinion in a form a judge can rely on. A well-evidenced diminution figure often helps the parties settle before a hearing.
When you might need this
Typical instructions include a buyer or owner pursuing a claim for damage or a hidden defect, a neighbour affected by construction or a right-to-light issue, and solicitors who need an independent valuation to quantify a client’s loss. We prepare these assessments for property across Canterbury and Kent.
Is diminution in value the same as the cost of repairs?
Not always. Sometimes the loss is close to the repair cost, but often it is different — repairs may cost more than the value they restore, or a residual stigma may reduce value even after the work is done. The court is usually interested in the loss of value, which is why a valuer’s assessment is needed rather than just a builder’s quote.
Who can give expert witness evidence on value?
Valuation evidence should come from a chartered surveyor who is an RICS member and, for formal valuations, a valuer on the RICS Register of Valuers, experienced in the type of property and the kind of dispute. That combination is what gives the report weight.
Get a fixed-fee quote. Canterbury Surveyors provides RICS surveys and valuations across Canterbury and the wider Kent area. Call 0204 579 8270 or use the enquiry form, and we will confirm the fee and the earliest inspection date.