Extending your home is one of the most effective ways to create additional living space and increase property value without the stress and expense of moving house. Whether you are planning a rear extension, side return, or wrap-around extension, understanding the likely costs is an essential first step.
In this 2026 guide, we break down realistic house extension costs in London, the key factors that influence pricing, and what homeowners should expect when planning a project.
Average House Extension Costs in London (2026)
The cost of a house extension in London varies depending on the size, design complexity, structural requirements, and level of finish. However, the following figures provide a realistic guide for most residential projects.
Rear Extension
Typical Cost: £60,000 – £90,000
Rear extensions remain one of the most popular choices for London homes, particularly for creating larger kitchens, open-plan living spaces, or dining areas with direct garden access.
Wrap-Around Extension
Typical Cost: £80,000 – £120,000+
A wrap-around extension combines a rear extension with a side return extension to maximise internal space. These projects are more structurally complex and generally involve higher construction costs.
Average Cost Per m²
Typical Range: £1,800 – £3,000 per m²
The final cost per square metre depends heavily on specification and complexity:
- Lower range: simpler layouts and standard finishes
- Higher range: bespoke design, large glazing systems, premium materials, complex steelwork, and high-end interiors.
What Affects the Cost of a House Extension?
Every property is different, and several factors can significantly influence the overall budget.
1. Size and Structural Complexity
Larger extensions naturally require more materials and labour. Costs also increase when projects involve:
- Structural steel beams
- Open-plan layouts
- Removal of load-bearing walls
- Complex roof structures
Victorian and terraced properties in London often require additional structural considerations.
2. Ground Conditions and Foundations
Foundation requirements can vary substantially depending on site conditions.
Additional costs may arise from:
- Poor soil conditions
- Nearby trees
- Existing drains beneath the extension
- Deep foundations required by Building Control
Drainage relocation alone can add several thousand pounds to a project.
3. Specification and Interior Finishes
The level of finish has a major impact on the final budget.
Key cost variables include:
- Kitchen specification
- Flooring materials
- Aluminium or bespoke glazing systems
- Underfloor heating
- Lighting design
- Joinery and storage solutions
Upgrading finishes can easily increase project costs by £10,000–£30,000 or more.
4. Planning, Design and Structural Calculations
Professional fees are an important part of the overall budget and are often underestimated by homeowners.
Typical costs include:
- Structural engineer: approximately £400–£1,000
- Architectural drawings
- Planning applications (where required)
- Building Control fees
A properly planned project reduces delays, unexpected costs, and construction issues later on.
5. Property Access and Logistics
Access is a major consideration on London projects, particularly with terraced houses.
Limited access may require:
- Additional labour
- Manual material handling
- Special delivery arrangements
- More extensive site protection
This can increase both labour costs and project duration.
Example London House Extension Project
Below is an example of a typical rear extension project completed in South London.
Project Overview
- Location: South London
- Extension Size: Approximately 25m²
- Project Duration: Around 12–14 weeks
Cost Breakdown
- Groundworks & foundations — approximately £12,000
- Structure & brickwork — approximately £18,000
- Roofing & insulation — approximately £9,000
- Aluminium windows & doors — approximately £8,000
- Electrical & plumbing works — approximately £6,000
- Plastering & internal finishes — approximately £7,000
Total Project Cost
£60,000
Is a House Extension Worth It?
In many cases, a well-designed extension is a strong long-term investment.
Benefits often include:
- Increased property value
- Improved layout and functionality
- Avoiding the high cost of moving home
- Creating modern open-plan living space
- Better use of existing property footprint
In London, a high-quality extension can often increase property value by approximately 15–25%, depending on the location and design.
Common Mistakes Homeowners Make
Underestimating the Budget
Many homeowners focus only on construction costs and forget to account for design fees, permissions, finishes, and contingency budgets.
Choosing the Cheapest Quote
A low quote is not always good value. Poor workmanship, unclear scope, and hidden extras often cost more in the long run.
Poor Space Planning
A larger extension does not automatically mean a better layout. Thoughtful design is essential for creating practical living space.
Ignoring Structural and Technical Details
Proper structural calculations and detailed planning are critical for avoiding delays and costly changes during construction.
Get a Tailored Quote Within 24 Hours
Every extension project is different, and the most accurate way to understand costs is through a tailored assessment.
To receive a preliminary estimate, simply fill in the quote request form below.
