If you’re 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 a house extension can vary significantly depending on the size of the extension, the complexity of the design and the quality of the finishes. Construction costs in London are typically higher than elsewhere in the UK, so it’s important to understand the likely budget before starting your project.
This guide explains the typical cost of a house extension in London, the factors that influence the overall budget and the additional costs you should plan for.
Average Cost of a House Extension in London
In London, most house extensions currently cost between £2,500 and £4,000 per square metre, depending on the specification and complexity of the build.
As a guide:
- A modest single‑storey rear extension may cost £60,000–£100,000.
- A larger extension with premium finishes or complex structural work can cost considerably more.
- Double‑storey extensions often offer better value per square metre, although the total project cost will naturally be higher.
These figures generally relate to construction costs only and may not include professional fees, planning application fees, Building Regulations costs or VAT.
Construction prices can fluctuate, so it’s always advisable to obtain several quotations before committing to a contractor.
Factors That Affect Extension Costs
Size of the Extension
Larger extensions cost more overall, although the cost per square metre often reduces slightly as the size of the project increases.
Structural Complexity
Projects requiring steel beams, significant structural alterations, difficult ground conditions or reinforced foundations will usually cost more than straightforward extensions.
Kitchens and Bathrooms
Adding a kitchen, utility room or bathroom can significantly increase costs because of plumbing, electrical work, appliances and specialist finishes.
Glazing and Doors
Large sliding doors, rooflights and glazed extensions can substantially increase the overall project cost compared with traditional masonry construction.
Interior Finishes
Floor finishes, bespoke joinery, lighting, kitchens and decorating can vary enormously in price depending on the specification chosen.
Professional Fees
In addition to construction costs, most extension projects involve several professional services, including:
- Measured building survey
- Planning drawings
- Planning application
- Building Regulations drawings
- Structural engineer’s calculations
If you’re planning an extension, our Planning Drawings service explains how we prepare planning applications for homeowners across London and the South.
Planning Permission Costs
Some house extensions can be built under Permitted Development Rights, meaning full planning permission may not be required. Larger extensions, properties within conservation areas and listed buildings often require a planning application. The current planning application fee for a householder application in England is set by the Government. The latest fees and guidance are available on the Planning Portal.
Even where planning permission is not required, many homeowners choose to apply for a Lawful Development Certificate to provide formal confirmation that the work is lawful.
Building Regulations
Regardless of whether planning permission is required, almost all extensions must comply with the Building Regulations, covering structural safety, fire safety, thermal insulation, ventilation, drainage and energy efficiency. Most extensions require detailed Building Regulations Drawings before construction begins.
Typical Extension Types in London
Common house extensions include rear extensions, side return extensions, wraparound extensions, double‑storey extensions and loft conversions. Terraced houses often benefit from side return extensions that transform narrow kitchens into large open‑plan living spaces. Semi‑detached and detached properties frequently have greater scope for larger rear or two‑storey extensions.
Other Costs to Consider
Homeowners should also budget for structural engineer’s fees, Party Wall Surveyor (where applicable), Building Control fees, Thames Water Build Over Agreements (if required), landscaping, kitchen and bathroom installation, decorating and a contingency of around 10–15% of the construction cost.
Final Thoughts
House extension costs in London vary considerably depending on the size, complexity and specification of the project. Understanding the likely costs before starting the design process will help you set a realistic budget, compare builder quotations more effectively and reduce the risk of unexpected expenses during construction.
Investing in good design and detailed technical drawings at the outset often saves far more money during the build itself.
Need Planning or Building Regulations Drawings?
If you’re considering extending your home, our Planning Drawings service can help you prepare a planning application that gives your project the best chance of success.
Once planning permission has been granted, our Building Regulations Drawings service provides the detailed technical drawings required for Building Control approval and construction.
If you’re unsure whether your extension requires planning permission, our guide Do I Need Planning Permission? Your Complete Guide for England explains the rules, permitted development rights and when an application is required.
If you’ve already received planning permission, our article What Happens After Planning Permission Is Approved? explains the next steps before construction can begin.
Ready to discuss your extension? Contact us today for a free, no‑obligation quotation.
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Construction costs vary depending on location, specification and market conditions. Always obtain professional advice and quotations before commencing a building project.