How Much Does a House Extension Cost in London?
If you’re a homeowner thinking about extending or refurbishing your property, one of the first questions that naturally comes up is one of costs. So how much does an extension actually cost?
It’s a good question, but one that doesn’t have a single, simple answer. Costs will vary depending on the type of project, the character of your home, the standard of finishes, and the quality of the builder.
If you want some baseline costs that you can use to get started budgeting and planning your project, read on below.
Outline Cost Breakdown
The three most common categories for home extensions are rear or side extension, loft conversion, and internal refurbishment. An extension requires ground works and new structure. A loft conversion requires insulation and fabric upgrades. An internal refurbishment can involve anything from upgrading finishes, to extensive remodelling.
These figures offer a helpful benchmark, but it’s important to remember that no two projects are the same. Site conditions, access, and design choices all play a big role in shaping the final cost.
| Type of Project | Typical Cost (per m²) | Description |
|---|---|---|
| Rear or Side Extension | £2,500 – £3,500 / m² | Includes new kitchen or living area extensions with full finishes. |
| Loft Conversion | £2,000 – £2,500 / m² | Suitable for adding an extra bedroom, ensuite, or home office. |
| Internal Refurbishment | £1,200 – £2,500 / m² | Covers reconfiguring existing layouts, new finishes, and minor structural changes. |
Construction costs is generally quoted ex VAT, so this will need to be added to the figures above. For normal extensions, loft conversions and refurbishments, add 20% VAT. If the work involves creating new units in an existing building, it is normally 5%. New builds, however, are currently zero-rated.
You will also need to factor in professional fees for the architect, engineers and surveyors. Depending on the complexity and the scope of their involvement, this usually range between 10-20% of the construction cost (ex VAT).
Why Costs Vary So Much
London homes are wonderfully diverse, and range from Victorian terraces and Georgian townhouses to modern flats, and each comes with its own set of opportunities and constraints.
A larger straightforward extension at the rear of a detached home will usually cost less per square metre than a smaller extension, built in a tight rear garden of a terraced house, where materials must be carried through the house.
The level of finish can also make a significant difference. High-end glazing systems, specialist finishes, and expensive materials can elevate the result, but will also add a fair amount to the cost.
Similarly, if your home is listed or in a conservation area, you may need additional permissions or structural input, which affects timelines and fees.
Sustainable upgrades, such as extra insulation, air-source heat pump, and natural materials, could also cost more than the standard – but these are upgrades we really think you should consider!
Design complexity matters too. A well considered scheme can make big impacts on the result without making too many big changes to the building. Often, structural changes can be omitted by looking at ways to work with what is already there. The key is balancing ambition with practicality. That way we can create a design that adds real value without unnecessary expense and an inflated carbon footprint.
The Value of Professional Guidance
As architects with years of experience in residential extensions and refurbishments across London, we work with homeowners from the very first sketch through to planning approval and construction.
One of our first steps is to establish a realistic budget and help you understand how your aspirations translate into cost, get a sense of what’s possible — and where to prioritise your investment.
At first, costs are based on experience from similar past projects. As the project progresses, more detail is added to the design, which translates into more detailed understanding of costs.
We recommend engaging with a builder early on. This is the best way to get cost certainty. From years of working on extension projects, we have good connections with excellent builders across London.
Talk to Us About Your Project
Every home, site, and family is different — and so are the costs involved.
If you’d like to get a better understanding of what your project might cost, we’d be happy to talk it through and share how certainty around budget can be gained at various stages of the project before all the decision are made.
Our team can review your property, discuss your goals, and provide a realistic estimate based on your specific circumstances.
There’s no obligation — just straightforward, practical advice from an experienced architectural practice that understands how to make the most of London homes.
Contact us today to arrange an initial consultation and start planning your home extension or refurbishment with confidence.
