How Much Does a House Extension Cost in London? (2026 Guide)
If you are considering extending your home, one of the first questions most homeowners ask is:
How much does a house extension cost in London?
The cost of an extension can vary widely depending on the size of the extension, the complexity of the design, and the quality of the finishes. London construction costs are typically higher than the rest of the UK, so it is important to understand the likely price range before starting a project.
This guide explains typical extension costs in London and the factors that influence the total project budget.
Average Cost of a House Extension in London
In London, most house extensions currently cost somewhere between £2,500 and £4,000 per square metre, depending on the specification and complexity of the build.
For example, a modest rear extension might cost between £60,000 and £100,000, while a larger extension with high-end finishes or structural complexity could cost significantly more.
These figures include construction costs but do not always include professional fees, planning costs, or VAT.
Construction prices fluctuate regularly, so it is always advisable to obtain quotes from contractors before committing to a project.
You can learn more about construction cost guidance from the UK Government here:
https://www.gov.uk/improve-energy-efficiency
Factors That Affect Extension Costs
Several factors influence the overall cost of a house extension.
Size of the Extension
Larger extensions obviously cost more overall, although the cost per square metre can sometimes decrease slightly as the project size increases.
Structural Complexity
Extensions that require significant structural work, steel beams, or foundation reinforcement will usually cost more than simple single-storey additions.
Kitchen and Bathroom Installations
If the extension includes a new kitchen, bathroom, or utility space, the cost can increase substantially due to plumbing, appliances, and specialist finishes.
Glazing and Doors
Large sliding doors, roof lights, and glazed extensions can significantly increase costs compared to standard wall construction.
Interior Finishes
Flooring, joinery, lighting, and decoration can vary greatly in price depending on the quality of materials selected.
Professional Fees
In addition to construction costs, most extension projects also involve professional design and approval costs.
These may include:
Measured building surveys
Planning drawings
Planning application submission
Building regulations drawings
Structural engineer calculations
If you need help preparing planning drawings for an extension, you can learn more about the services offered by Draw & Plan here:
https://www.drawandplan.com
Professional design fees are usually a relatively small percentage of the overall construction cost but are essential to ensure the project is properly designed and approved.
Planning Permission Costs
Some house extensions can be built under permitted development rights, meaning full planning permission may not be required.
However, larger extensions or projects in conservation areas may require a planning application.
The current planning application fee for householder applications in England is set by the government and is typically around £258.
You can learn more about the planning application process here:
https://www.planningportal.co.uk/applications/how-to-apply
Even when planning permission is not required, many homeowners choose to apply for a Lawful Development Certificate to confirm the project is permitted.
Building Regulations
Regardless of whether planning permission is required, most extensions must comply with building regulations.
Building regulations ensure that the construction meets standards for structural safety, insulation, fire safety, and drainage.
Detailed construction drawings are normally required for building regulations approval before work begins.
You can read more about building regulations here:
https://www.gov.uk/building-regulations-approval
Typical Extension Types in London
Some of the most common extension types in London include rear extensions, side return extensions, wraparound extensions, and loft conversions.
Terraced houses in London often benefit from side return extensions, which can dramatically improve kitchen and living spaces.
Semi-detached and detached houses may have more space for larger rear extensions or multi-storey additions.
Each property type presents different design opportunities and planning considerations.
Final Thoughts
House extension costs in London vary depending on the size and complexity of the project, but most homeowners should expect construction costs in the region of £2,500 to £4,000 per square metre.
Understanding the likely cost range before starting the design process can help you plan your project realistically and avoid unexpected expenses.
If you are considering extending your home and need planning drawings or building regulations drawings, you can learn more about the services offered by Draw & Plan here:
https://www.drawandplan.com
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Always consult relevant professionals and local authorities before undertaking any development or change of use
