House Extension Cost UK
Building a house extension is one of the most significant home improvement projects UK homeowners undertake. Whether you need more kitchen space, an extra bedroom, or a home office, understanding the true costs helps you plan effectively and avoid budget surprises.
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Get Your Free EstimateUK House Extension Costs (2026)
Single Storey (per m²)
£1,400 - £2,500
Basic to high-end finish
Two Storey (per m²)
£1,200 - £2,000
Better value per m²
Small Extension (15-20m²)
£30,000 - £50,000
Typical kitchen extension
Large Extension (30m²+)
£55,000 - £100,000+
Open-plan living space
Types of House Extension
Single-Storey Rear Extension
The most popular extension type, typically used to create larger kitchen-diners or family rooms. Costs range from £40,000-£80,000 for a 20-25m² extension. Many fall under permitted development, avoiding planning delays.
Two-Storey Extension
Offers excellent value per square metre by adding space on two floors. Expect to pay £60,000-£150,000 for 40-60m² of additional space. Planning permission is usually required.
Side Return Extension
Popular for Victorian and Edwardian terraces, using the narrow alley alongside the kitchen. Costs £30,000-£60,000 and can significantly improve kitchen size and layout.
Wrap-Around Extension
Combines side and rear extensions for maximum impact. Expect to pay £60,000-£120,000 for a single-storey wrap-around, creating substantial open-plan living space.
What Affects Extension Costs?
- Size and complexity: Larger, simpler extensions cost less per m²
- Foundations: Poor ground conditions can add £5,000-£20,000
- Roof type: Flat roofs are cheaper than pitched, tiled, or lantern roofs
- Glazing: Bi-fold doors, skylights, and large windows increase costs
- Internal specification: Underfloor heating, premium flooring, high-end finishes
- Kitchen/bathroom fitting: Often excluded from base extension quotes
- Location: London and the South East are 25-40% more expensive
Additional Costs to Budget For
| Item | Typical Cost |
|---|---|
| Architectural drawings | £1,500-£5,000 |
| Structural engineer | £500-£1,500 |
| Planning application | £206 (householder) |
| Building regulations | £500-£1,500 |
| Party wall agreements | £1,000-£2,000 per neighbour |
Professional design fees vary depending on the scope and level of support required. You can estimate them separately with the UK architect cost calculator.
Calculate Your Extension Cost
Get a personalised extension cost estimate using our free renovation calculator. Enter your property details, extension type, and location to receive realistic cost ranges.
Use your estimate as a starting point when obtaining quotes from builders. We recommend getting at least three detailed quotes and comparing them carefully before committing.
How We Calculate Costs
RenoCost estimates are calculated from published UK cost benchmarks and RenoCost's documented per-module cost assumptions. The model adjusts these assumptions using the property characteristics, selected renovation scope, finish level and broad postcode-based pricing region, then presents a low, mid and high planning range.
- Published UK benchmarks — macro reference points for construction costs and materials and labour trends, used to sense-check RenoCost's own assumptions.
- RenoCost's per-module assumptions — cost and duration assumptions for each renovation module, reviewed and updated periodically by the RenoCost team.
- Estimate ranges — low, mid and high bands set to reflect the realistic spread between competitive and premium specifications, rather than a single fixed price.
Data confidence note
Completed calculator estimates are used to produce anonymous renovation-planning insights. They are not contractor quotations, invoices or confirmed final project costs. RenoCost estimates are intended for initial planning and budgeting. Property condition, specification, access, contractor availability and site-specific requirements can change the final cost. Always obtain detailed written quotations before committing to a project.
Regional adjustments are applied based on postcode area to reflect broad differences in local labour and material rates. See our Method Glossary for definitions of the terms used here.