How much does a house extension cost in London?

House extension costs in London vary depending on size, structural complexity and specification. As a general guide, many projects begin from around £3,000 per square metre, with higher-specification or more complex extensions costing more.

How much does a loft conversion cost in London?

Loft conversions in London often vary widely depending on the type of conversion, roof structure, access requirements and finishes. Many projects fall within a broad range from around £65,000 upwards.

Why do construction costs vary so much?

Construction costs vary due to factors such as structural requirements, site access, planning constraints, drainage, glazing systems, kitchens, bathrooms and the quality of finishes selected.

Construction Costs in London: What Homeowners Should Expect

Q: Are online construction cost guides fixed quotations?

No. Online construction cost guides are general guidance only. Accurate pricing always depends on the design, specification, structural requirements and site conditions of the individual project.

January 22, 2024
By marianna

Planning a house extension, loft conversion, renovation or new build usually begins with one important question — how much will it cost?

Construction costs in London can vary significantly depending on the type of project, the size of the build, the structural requirements and the level of finish you want to achieve. Factors such as access, planning constraints, existing property condition and material choices all play a role in the final price.

At S & M Solutions Ltd, we regularly help homeowners understand these cost variables early in the process so projects can move forward with clearer expectations, better planning and more realistic budgets.

Understanding how construction costs are formed is one of the best ways to avoid surprises later and make better decisions from the outset.

Completed loft refurbishment and home renovation project in Blackheath SE3

Why Construction Pricing Can Be Difficult

Many builders can provide broad budget figures for the early stages of a project, particularly for structural work, roofing and first fix installations. However, pricing becomes more complex once detailed specifications and finishes are involved.

Items such as kitchens, bathrooms, glazing, flooring, joinery, lighting and heating systems can vary greatly in cost depending on the products selected. This applies just as much to house extensions as it does to loft conversions, refurbishments and new builds.

When these decisions are left until construction is already underway, progress on site can slow down while final choices are made. This often affects sequencing, labour efficiency and the overall smooth running of the project.

For this reason, experienced builders usually recommend agreeing as many specifications as possible before work begins, helping the build progress more efficiently and with fewer delays.

House Extensions and Cost Planning

House extensions are one of the most popular ways to improve space and add value to a home, but costs can vary widely depending on the type of extension being built.

A simple rear extension is generally more straightforward to price than a side return or wrap-around extension, particularly where structural alterations, drainage changes or complex steelwork are involved. Double-storey extensions also require careful consideration of foundations, load paths and roof integration, all of which influence the final cost.

In London, the condition and layout of the existing property also matter. Period homes often require more structural coordination than modern houses, and restricted access can increase labour time and logistical costs.

For this reason, extensions are best planned with a clear understanding of layout, specification and structural requirements from an early stage.

Modern kitchen and dining space created during a residential wrap around extension and renovation project by S & M Solutions Ltd in Blackheath, London SE3.

Renovation Projects and Hidden Variables

Renovation and refurbishment works present additional challenges when estimating costs.

Unlike new builds or straightforward extensions, existing properties can conceal structural and service-related issues that only become visible once work starts. Older homes in London frequently require upgrades to electrics, plumbing, insulation or structural elements to bring them in line with modern standards.

This is why experienced contractors often recommend allowing a contingency within the budget, particularly for refurbishment projects involving significant internal alterations.

Early surveys, exploratory works and realistic planning can all help improve cost accuracy before construction begins.

Typical Construction Cost Guidelines in London

While every project is different, the following figures provide a general indication of construction costs in London based on current market conditions.

House Extensions

In London, many house extensions currently fall within the range of £3000 to £5000 per square metre, depending on the type of extension, structural complexity and specification. Smaller extensions can sometimes cost more per square metre , while high-specification projects will exceed this range.

Loft Conversions

Loft conversions in London often range between £65,000 and £120,000 or more, depending on the design, roof structure, access requirements and internal finishes. Dormer loft conversions, mansard conversions and projects involving significant structural work typically sit at the higher end of the range.

Renovations and Refurbishments

Renovation costs vary greatly depending on the age and condition of the property, the amount of structural change involved and the quality of finishes selected. Full refurbishments in London can increase substantially where rewiring, replumbing, insulation upgrades and internal reconfiguration are required.

New Build Homes

A typical two-storey family house in London generally costs between £2,800 and £3,500 per square metre for standard construction and finishes. Higher-specification homes or projects with complex structural requirements may exceed £4,000 per square metre.

Garden Rooms and Outbuildings

A cavity-wall garden office, gym or studio typically starts from £2,400 to £3,000 per square metre, depending on glazing, insulation, heating and electrical installations.

Why Planning Ahead Helps Control Costs

One of the most effective ways to manage construction costs is through careful planning and early collaboration between the client, designer and builder.

This is especially important on extensions and loft conversions, where structural decisions, layout changes and finish selections can significantly affect cost if left unresolved. When the main design decisions are agreed in advance, the project is usually easier to price accurately and deliver more efficiently.

A design and build approach can simplify the process further by bringing design, technical coordination and construction together under one team. This helps identify problems earlier, reduce uncertainty and improve overall cost control.

Final Thoughts

Whether you are planning a house extension, loft conversion, home renovation or new build, the key to managing construction costs is realistic early planning.

The final price of any project will depend on a combination of factors including design complexity, structural requirements, site constraints and the level of finish you want to achieve. Understanding these variables at the start helps projects move forward with greater clarity and fewer costly surprises later on.

For homeowners considering building work in London, early discussions with an experienced contractor can provide valuable guidance on likely costs, practical options and the right route forward.

Important Note

The cost ranges in this guide are intended as general guidance only. Accurate pricing always depends on the design, specification, structural requirements and site conditions of each individual project.