How much does a 40m2 extension cost in the UK? It’s a question many homeowners ask when planning to expand their living space. Whether you’re building a kitchen extension, adding a rear room, or creating a larger open-plan area, understanding the cost implications of a 40-square-metre extension is vital for budgeting. At KT5 Construction, we specialise in high-quality residential extensions across London and Surrey and are here to break down what you can expect to pay — and why.

You can also use our Free Extension Cost Calculator to get an instant quote in under 60 seconds.

Average Cost of a 40m2 Extension in the UK

Key Factors That Influence Pricing

The cost of a 40m2 extension in the UK typically ranges from:

• • £60,000 to £95,000 for a basic finish
  • £95,000 to £120,000 for a mid-range specification
  • £120,000 to £160,000+ for high-end or bespoke finishes

These figures include construction, materials, labour, and professional fees. However, prices vary based on:

• • Type of extension (rear, side return, wraparound, double-storey)
  • Ground conditions and foundation requirements
  • Structural complexity (steelwork, load-bearing walls)
  • Roof type (flat roof, pitched roof, roof lights)
  • Internal finishes (flooring, kitchen, bathrooms, utilities)
  • Location (London and South East generally cost more)

Please note: These prices may vary significantly based on your choice of materials, design specifications, and unique site factors.

What Does a 40m2 Extension Include?

A 40m2 extension is approximately the size of a generous kitchen-diner or two medium-sized rooms. Common inclusions are:

• • Foundations and groundwork
  • Brickwork and structural supports
  • Roofing and insulation
  • Windows, bi-fold or sliding doors
  • First-fix and second-fix electrics and plumbing
  • Plastering and decoration
  • Flooring and lighting

Optional upgrades: underfloor heating, bespoke joinery, kitchen installation, or smart home integration.

Why Choose KT5 Construction for Your Home Extension?

• • Over 25 years of experience delivering high-end extensions in London and Surrey
  • Dedicated project managers and daily site supervision
  • Transparent, itemised pricing with no hidden costs
  • In-house Groundworks specialists for precision foundations
  • Design & build expertise from concept to completion

Whether you’re planning a kitchen extension or a complete home transformation, KT5 Construction has the experience and attention to detail that ensures your 40m2 extension adds real value.

Cost Breakdown by Category

Groundworks and Foundations

Depending on soil type and access, groundwork costs can range from £8,000 to £15,000. KT5 Construction’s Groundworks team ensures accuracy and compliance with building regulations from day one.

Structure and Shell

Expect to budget between £30,000 to £50,000 for walls, roofing, and windows. We use high-quality materials that meet or exceed British Standards.

Internal Fit-Out

From electricals and plumbing to plaster and paint, internal works may cost £20,000 to £40,000, depending on your chosen finishes.

Professional Fees

Architectural design, structural engineering, and planning approval can add another £8,000 to £12,000. We offer a full design & build service to streamline this process.

FAQs About 40m2 Extension Costs

Do I need planning permission for a 40m2 extension?

In many cases, a 40m2 rear extension may fall under permitted development rights but always check with your local authority. KT5 Construction can advise you and handle applications if needed.

How long does it take to build a 40m2 extension?

Typically 10–16 weeks depending on complexity, weather, and finishes.

Can I reduce costs without compromising quality?

Yes. Choosing simpler finishes, reusing existing materials, and limiting structural alterations can save money. KT5 offers itemised quotes to help you make informed decisions.

Ready to Build Your Extension?

Wondering exactly how much your 40m2 extension will cost in the UK? Contact KT5 Construction today for a free, no-obligation consultation.