PropeteerProperty intelligence

Property value guide

How much value does an extension add?

An extension adds the value buyers place on the larger, better-functioning completed home—not its build cost pound for pound. Compare sales of the target size and room mix, then account for layout, quality, planning and the local ceiling.

Written and reviewed by Propeteer Research TeamPublished Reviewed

At a glance

Key facts

Primary valuation
Completed target-property evidence
Cross-check
Local £/sq ft or £/m²
Cap
Local ceiling and buyer demand

A reliable process

How to approach the question.

  1. 01Define existing and completed floor area, room mix and condition.
  2. 02Find sold homes resembling the completed result.
  3. 03Cross-check incremental area without double-counting bedrooms or bathrooms.
  4. 04Deduct all-in costs and apply planning, layout and evidence-quality controls.

Worked example

A 65 m² home expanded to 165 m²

A small percentage uplift on the original home is inappropriate. The central question is what credible 165 m² homes of the new type and finish sell for locally, subject to the street’s ceiling and whether the design is deliverable.

What this does not prove

  • Added area can have diminishing value per square foot.
  • Poor layout or loss of garden can offset part of the gain.
  • Cost, value and planning feasibility are separate outputs.

Method used

  • Resolve the property identity before joining sales, certificates, floor area and local market evidence.
  • Prefer recent completed sales of genuinely comparable homes and explain each adjustment rather than averaging every nearby transaction.
  • Use a range and reduce confidence where the evidence is sparse, contradictory or dependent on unverified property characteristics.

Primary sources and official guidance

Price Paid DataHM Land RegistryEnergy Performance of Buildings DataDepartment for Levelling Up, Housing and CommunitiesUK House Price IndexHM Land Registry, ONS and partnersLocal planning authority registersUK local planning authorities

Source scope, licensing, refresh expectations and known limitations are listed in the public data-source register.

Related guides

What is a local property ceiling price?

Use the strongest nearby completed sales to test whether additional spend or floor area is likely to be fully reflected in resale value.

Renovation value versus cost

Separate construction cost, gross value created and net equity while accounting for condition, added space, room function and local evidence.

Apply this guide

Start with the exact UK address.

Research a property