House extension cost guide: What to budget
Extending your home is one of the most popular ways to create more space without moving. Whether you’re planning a rear extension, side return or double-storey build, understanding the likely costs will help you budget properly and compare builder quotes with confidence.
Average house extension costs in the UK
Extension costs vary depending on size, design complexity, materials and location. A common way to estimate cost is by price per square metre.
As a general guide:
- Basic single-storey extension: £1,800–£2,500 per m²
- Mid-range extension: £2,500–£3,000 per m²
- High-spec extension: £3,000–£4,000+ per m²
For example:
- Small 20m² extension: £36,000–£60,000+
- Medium 30m² extension: £54,000–£90,000+
- Larger double-storey extension: £60,000–£120,000+
London and the South East typically sit at the higher end of these ranges.
What affects extension costs?
Several key factors influence the total price, including size and structure. The larger the extension, the higher the material and labour costs. Double-storey extensions can be more cost-effective per m² but require stronger foundations.
Foundations and ground conditions, poor soil conditions or drainage issues may increase groundwork costs.
Design and complexity; simple rectangular designs cost less than complex layouts with large glazing, roof lanterns or structural steel. Don't forget to consider the costs of your interior finish. Kitchen extensions, underfloor heating, bifold doors and premium finishes will raise the overall budget.
Professional fees
Architects, structural engineers, planning applications and building control fees can add several thousand pounds to the total.
Additional costs to consider
Beyond the build itself, homeowners should budget for:
- Planning permission (if required)
- Building regulations approval
- Structural calculations
- Party wall agreements
- Decorating and landscaping after completion
These costs can add 10–20% to your overall project budget.
Ways to manage extension costs
Keeping the design simple, avoiding unnecessary layout changes, and selecting mid-range finishes can help control spending. Getting detailed quotes from multiple local builders ensures you understand exactly what is included.
Final thoughts
A house extension is a major investment, but it can significantly increase your living space and property value. Understanding cost per square metre, design impact and additional fees will help you budget realistically and choose the right builder for your project.