Extension Cost Calculator UK
Planning a home extension is one of the most significant investments UK homeowners make. Our extension cost calculator helps you understand the likely costs before you commit, giving you a realistic budget range based on current UK construction prices.
Estimate your extension cost with our free calculator
Get Your Free EstimateAverage UK Extension Costs (2026)
Single Storey Extension
£1,400 - £2,200/m²
Two Storey Extension
£1,200 - £1,800/m²
Side Return Extension
£30,000 - £60,000
Rear Extension (4x5m)
£40,000 - £80,000
How Our Extension Calculator Works
Our calculator uses published UK cost benchmarks and RenoCost's own per-module cost assumptions, adjusted for regional pricing, to generate a planning estimate. Simply enter your extension type, size, location, and finish level to receive a personalised cost range.
The calculator accounts for regional price variations across the UK, with costs in London and the South East typically 25-40% higher than in Scotland, Wales, or northern England.
What Affects Extension Costs?
- Size and complexity: Larger extensions cost less per square metre but more overall
- Foundation requirements: Poor ground conditions can add £5,000-£15,000
- Roof type: Flat roofs cost less than pitched or tiled options
- Glazing: Bi-fold doors and skylights add £3,000-£10,000
- Heating and electrics: Extending existing systems vs new installations
- Internal finish: Basic, mid-range, or high-end specifications
- Access: Difficult access can increase labour and material delivery costs
What Is Typically Included?
A standard extension quote should include foundations, walls, roof, windows and doors, basic electrics and plumbing, plastering, and flooring. However, quotes vary significantly in what they cover.
Items often excluded include kitchen or bathroom fitting, decorating, landscaping, planning fees, building regulations, and structural engineer fees. Always clarify exactly what is included before accepting a quote.
Architectural drawings, planning support and technical design are often priced separately from the main build cost. You can estimate these using the architect fees calculator.
Regional Cost Variations
Extension costs vary significantly across the UK. London commands the highest prices, often 40% above the national average. The South East follows at around 20% above average. The Midlands, North, Scotland, and Wales typically fall at or below the national average, making extensions more affordable in these regions.
Getting an Accurate Estimate
While our calculator provides a useful starting point for planning, the only way to confirm a final cost is with professional quotes. Use RenoCost to understand the likely cost range, then obtain detailed quotes from at least three local builders to refine your budget.
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.