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.

How Our Extension Calculator Works

Our calculator uses RenoCost's own per-module cost assumptions, informed by published UK cost benchmarks and adjusted for regional pricing, to generate a planning estimate. For an extension, the journey is:

1. Property: choose your property type, number of bedrooms, property age and current condition, and enter a valid UK postcode.
2. Work: select House Extension, then choose single or double storey in a small, medium or large size (roughly 15m², 25m² or 40m² for single storey; 40m², 60m² or 80m² in total for double storey). You can add other work, such as a kitchen, at the same time.
3. Finish level: choose the specification level you expect.
4. Results: enter your name and email to view the estimate on screen. You see a low-to-high range with a central figure, a range for each work item, an indicative timescale, and a contingency shown separately based on the property condition you chose.

The calculator does not take an exact floor area, roof type, glazing choice or ground survey. Those are covered in the factors below and should be priced in contractor quotes. VAT and professional fees are not itemised, so confirm how they are treated in each quote.

The figures on this page and the extension cost guide are broad editorial ranges and may use different scope assumptions, such as whether glazing, kitchens or finishes are included. They are not calculator outputs. Your calculator result reflects the options you select.

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.