House Extension Cost Guide 2025

Complete UK price breakdown for all extension types

House Extension Costs by Type

  • Single-storey rear extension: £35,000 – £55,000 (for 20m²)
  • Two-storey extension: £70,000 – £120,000
  • Side return extension: £40,000 – £70,000
  • Over-structure extension: £60,000 – £100,000 (above existing single-storey)
  • Kitchen extension: £30,000 – £60,000 (typically single-storey with open-plan kitchen)

Regional Variation

Location significantly affects cost. London and Southeast England are typically 20-30% above national average due to higher labour costs and more complex site conditions. Rural areas may be cheaper but with longer material delivery times.

What Affects Extension Cost

  • Size: Larger extensions offer better value per m² but higher total cost
  • Specification: High-end finishes (designer kitchens, premium bathrooms) dramatically increase costs
  • Build type: Brick is standard but expensive; timber frame is faster and often cheaper
  • Existing foundations: Good existing foundations reduce costs; weak soil requires additional work
  • Ground conditions: Clay soil, high water table, or sloping sites increase costs
  • Architect fees: Typically £2,000-£6,000 for extension design
  • Planning fees: £206 for householder planning application

Cost Per Square Metre Guide

  • Standard specification: £2,000 – £3,000/m²
  • High specification: £3,000 – £4,500/m²
  • Luxury/bespoke: £4,500+/m²

What's Typically NOT Included in Builder Quotes

Budget for these additional costs:

  • Architect fees: £2,000 – £6,000
  • Party wall surveyor: £700 – £1,500
  • Planning application fee: £206
  • Building control fees: £200 – £500
  • Interior fit-out (beyond basic): Variable
  • Landscaping: £2,000 – £10,000+

How to Get Accurate Quotes

Follow these steps for accurate pricing:

  1. Get detailed architectural drawings before quoting
  2. Request at least 3 quotes from different builders
  3. Ensure quotes cover the same scope
  4. Choose between fixed-price (predictable) or cost-plus (flexible but variable) contracts
  5. Check what's included and excluded in each quote

Return on Investment

Extensions typically add 11-15% to property value. For example, a £250,000 house could see its value increase by £27,500-£37,500 with a quality single-storey extension — often exceeding the build cost.

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