Understanding the true house extension cost in the UK is essential before committing to a build, especially in London where space, access, and regulations can significantly affect pricing. In 2026, house extension costs continue to vary depending on size, design complexity, location, and the quality of finishes. Knowing the average cost per square metre helps homeowners set realistic budgets and avoid unexpected expenses during construction.
For a single-storey extension, the average house extension cost per m² in the UK ranges from £1,800 to £3,000. This type of extension is often used for kitchen expansions, dining areas, or open-plan living spaces. Costs are influenced by groundwork requirements, structural steel installation, roof design, and glazing. In London, single-storey extensions typically sit at the higher end of this range due to labour rates and restricted site access.
Double-storey extensions offer better value per square metre but come with a higher overall budget. In 2026, the average house extension cost per m² for a double-storey extension ranges from £1,600 to £2,500. While foundations and roof costs are shared across two levels, additional structural work, staircases, and internal finishes increase the total project cost. Double-storey extensions are popular in suburban London areas where homeowners aim to maximise space without sacrificing garden size.
Regional price differences play a major role in overall house extension cost. Outside London and the South East, costs are generally lower due to reduced labour and material expenses. In Greater London, prices are often 15–30% higher, particularly for terraced properties where logistics, party wall agreements, and noise restrictions add complexity. Conservation areas and listed buildings can also increase costs due to design limitations and specialist construction methods.
Architect fees are another important consideration when calculating house extension cost. In the UK, architects typically charge between 5% and 12% of the total build cost, depending on the level of service provided. Fees may include concept design, planning drawings, technical details, and project coordination. In London, architectural input is especially valuable due to planning constraints and the need to maximise limited space efficiently.
Building regulations costs must also be included in your budget. These cover structural calculations, thermal performance, fire safety, drainage, and sound insulation compliance. Building control fees generally range from £500 to £1,500 depending on the size and complexity of the extension. Structural engineer fees, often required for steel beams and foundations, typically add an additional £500 to £2,000.
When planning a house extension, it is essential to account for hidden costs such as ground investigations, utility connections, temporary works, waste removal, and interior finishes. A contingency budget of at least 10% is recommended to cover unforeseen issues, especially in older UK and London properties.
A well-planned extension is a long-term investment that enhances both lifestyle and property value. By understanding the real house extension cost per m² and working with experienced professionals, homeowners can achieve a high-quality result that meets regulations, stays on budget, and adds lasting value to their home.
