How much will an extension cost?

If you're considering adding space to your home, one of the first questions you're likely to ask is: “How much does an extension cost?” While no two projects are the same, it's helpful to have a broad understanding of typical costs and the factors that influence them.

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As a general rule of thumb, the cost of building a home extension in the UK typically ranges between £2,000 and £4,000 per square metre, excluding VAT. This range covers most residential extension projects and includes standard finishes — but not items like kitchen units, white goods, bespoke joinery, or furniture.

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Websites such as Costmodelling.com provide more in depth information if you want to look at cost variations across different typologies and areas in the UK.

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Example: A 30m² single-storey extension could cost between £60,000 and £120,000, depending on specification and complexity.

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Keep in mind that these are ballpark figures. Real costs vary depending on a range of project-specific factors.

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Factors That Affect the Cost of an Extension

Several key elements can influence the final price of your extension project:

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• Location – Construction costs vary by region. Projects in London and the South East tend to be more expensive than in other parts of the UK.

• Size and shape – Simpler footprints are typically more economical to build than complex or irregular shapes.

• Quality of finishes – Materials, detailing, and level of craftsmanship all affect cost.

• Kitchen or bathroom included – These are the most expensive rooms to build due to plumbing, ventilation, and fittings.

• Structural changes – Removing load-bearing walls or raising rooflines adds complexity and cost.

• Special features – Renewable energy systems, underfloor heating, home automation, or bespoke glazing systems can significantly increase the budget.

• How hands-on you are – Managing parts of the project yourself can save costs, but also adds risk.

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For more detailed, region-specific estimates, sites like Costmodelling.com provide useful construction cost data across various building types and locations in the UK.

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Don’t Forget Professional Fees

In addition to construction costs, you’ll need to budget for professional and statutory fees, which typically add 10–15% to the overall cost. These can include:

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• Architectural fees

• Structural engineer

• M&E (mechanical and electrical) consultant

• Planning application fees

• Building regulations fees

• Party wall surveyors (if required)

• Measured or topographical surveys

• Specialist reports (e.g. tree surveys, ecology surveys, flood risk assessments)

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Summary

The true cost of a house extension depends on many variables — but a well-prepared budget, supported by professional guidance, will help avoid surprises later on.

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If you're unsure where to start, we can help you develop a realistic cost plan and guide you through the planning, design, and build process with full transparency.

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Thinking of extending your home?
Get in touch for a tailored consultation and let’s talk about how we can make your space work harder for you.​