How Much Does a House Extension Cost?

David Hepburn
February 9, 2025

Introduction

Extending your home is one of the most effective ways to add living space, boost property value, and tailor your environment to suit your lifestyle. However, “How much does a house extension cost?” is the first question on every homeowner’s mind. In this guide, we’ll break down the key factors influencing extension costs in the UK, provide ballpark figures, and share tips on how to manage your budget effectively. We’ll also reference authoritative sources to give you a clear, reliable overview.

Factors Affecting Extension Costs

Size and Scope
- The bigger the extension, the more materials and labour you’ll need.
- Extensions typically range from £20,000 to over £100,000, depending on size and complexity.
Type of Extension
- Single-Storey vs. Double-Storey: Double-storey extensions provide more space but can cost more due to additional structural requirements.
- Side Return or Rear Extension: Side returns can be narrower and sometimes less costly, while rear extensions often offer more flexibility in design.
Location
- Costs vary significantly between regions. Extensions in London and the South East typically command higher labour and material rates.
- Rural areas can see increased costs if specialised materials have to be sourced from afar or tradespeople must travel long distances.
Quality of Materials
- High-end finishes like luxury flooring, premium glazing, or bespoke joinery will increase costs.
- Opting for standard-grade materials can help control your budget.
Planning & Building Regulations
- Planning Portal provides an overview of when planning permission is required. Additional fees for permits, architectural drawings, and structural calculations can add to the project cost.
- Complying with local building regulations (e.g., insulation standards, fire safety measures) also influences the final budget.
Professional Fees
- Architects, structural engineers, and surveyors each charge for their services, which can amount to around 8-15% of your total construction costs—depending on the complexity of your project.
- For guidance on typical fees, refer to the Royal Institute of Chartered Surveyors (RICS).

Typical Cost Breakdown

Below is a rough cost breakdown for a standard single-storey rear extension in the UK, to give you a starting point:

Basic Build (Structure & Shell): Approx. £1,200–£1,500 per m²
Mid-Range Finish (Flooring, Kitchen Install, etc.): Additional £300–£500 per m²
Architect, Surveyor & Structural Engineer Fees: 8–15% of build cost
Planning & Building Control Fees: £200–£2,000+ (depending on complexity)
Contingency: 10–15% of total budget for unexpected costs

These numbers can vary significantly based on your project’s unique specifications, so it’s wise to get detailed quotes from reputable contractors and professional architects.

Tips for Keeping Extension Costs Under Control

Plan Early and Thoroughly
- A well-thought-out design minimises last-minute changes. You’ll save time and money by finalising your requirements before construction begins.
- Contact Hepburn Architects to discuss conceptual designs, feasibility studies, and how to optimise your layout for cost and efficiency.
Shop Around for Quotes
- Always get quotes from multiple builders and suppliers to compare prices. Ensure each quote includes labour, materials, and any additional services.
- Check references or look for reviews on trusted sites like Which? Trusted Traders for reputable contractors.
Consider a Modular or Prefabricated Extension
- Off-site construction can be more cost-effective and faster to install, especially if you’re dealing with tight deadlines or limited site access.
Use Quality Materials Wisely
- Splurge on elements that have a big visual or functional impact—like windows, doors, or insulation—but opt for cost-effective solutions elsewhere to keep your budget balanced.
Be Prepared for Contingencies
- Set aside 10–15% of your total budget for unforeseen expenses. This buffer can cover hidden issues like plumbing or foundation problems discovered mid-project.

Why Professional Guidance Is Essential

Navigating planning permission, building regulations, and structural design can be complex. Hiring an experienced architect not only ensures compliance but also helps you design a space that adds genuine value to your home. At Hepburn Architects, we’ve guided numerous clients through the extension process—from initial planning to final sign-off. Our expert team can help you:

Create a cost-effective design that maximises functionality and natural light.
Ensure compliance with local planning permissions and building regulations.
Coordinate with trusted contractors for smooth project execution.

External Resources for Further Reading

Planning Portal – Official UK Government site for planning and building regulations.
RICS – Royal Institute of Chartered Surveyors, offering guidance on construction costs and professional standards.
Which? – Independent consumer champion offering advice on hiring professionals and checking service quality.

Conclusion

The cost of a house extension varies based on numerous factors—size, scope, materials, and location. While it may seem daunting at first, careful planning and professional guidance can help you create a dream space without breaking the bank. If you’re considering an extension and want tailored advice, g et in touch with Hepburn Architects today.

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