RICS recommends considering value before extending and using relevant comparable evidence when assessing property value. [1] [2]
Ask two different questions
First ask: “Would this make the house work better for us?” Then ask: “What would someone else pay for the completed house?” These are different decisions. A room that is particularly useful to your family can justify expenditure even where a resale calculation does not show an equivalent increase.
For the financial comparison, use a present-day value of the house as it is and a present-day estimate assuming the specific extension is completed. Do not quietly include assumed future house-price growth in the estimated benefit of the building work.
Use genuinely comparable properties
Ask a valuer to explain the evidence rather than applying a percentage from a national home-improvement article. RICS identifies location, size, use, construction and tenure among the factors that distinguish properties, and advises assessing recent, relevant transactions rather than relying on a headline price alone. [2]
Give the valuer the proposed plans and specification. Ask them to consider the resulting accommodation, not simply the extra square metres. For example, ask whether the layout gains a genuinely usable room, retains practical circulation and leaves a garden appropriate to the finished home. These questions make the assumptions behind the estimate visible.
An asking price is not the same thing as a completed transaction. Where the evidence is thin or the proposed home is unusual, ask for the uncertainty to be explained rather than forcing a precise uplift. [2]
Compare the uplift with the whole budget
Use the complete project expenditure in your comparison, including the construction scope, professional services, approvals and any temporary arrangements. A costed schedule is more useful than a builder's headline figure with unspecified exclusions. RICS recommends fully designing significant extensions to obtain comparable quotations. [1]
Check the VAT basis. HMRC states that work to an existing house is normally standard-rated, subject to specific exceptions; the fact that an extension is newly built does not by itself make it a zero-rated new home. [3]
A practical calculation is: estimated completed value minus current value minus total project expenditure. This is a comparison tool, not a prediction or a valuation method in its own right.
Protect the usefulness of the finished asset
Keep the planning decision or lawful development evidence, agreed drawings, structural information and building control completion documentation together. LABC notes that a completion certificate is a document you will be asked for when selling. [4]
Choose the design with both everyday use and eventual sale in mind. Obtain a property-specific valuation rather than treating an extension as a guaranteed investment or assuming a London-wide value uplift.