The Mirion blog
House Extension Costs in London: A Practical Pricing Guide
A straightforward guide to what house extensions actually cost in Wembley, Harrow, Brent and nearby London areas — with real ranges, what affects the price and how to plan your budget.
What does a house extension cost in London?
The short answer: most single-storey extensions in North West London cost between £35,000 and £70,000+. But the real figure depends on what you are building, where you are starting from and the level of finish you want.
This guide breaks down the main cost factors so you can plan your budget with confidence — before you commit to anything.
What affects the price?
Size and layout. A simple rear extension costs less than an L-shaped wraparound. Bigger footprints mean more foundations, more materials and more labour.
Structural work. Removing a load-bearing wall, adding steel beams or underpinning adds cost. If your extension opens into an existing room, expect structural steelwork in the budget.
Specification. Bi-fold aluminium doors, underfloor heating, skylights and high-end kitchen fittings all add to the final figure. Standard spec keeps costs down without cutting corners.
Ground conditions. Clay soil, tree roots or existing drains can complicate foundations. We check this during the survey so there are no surprises later.
Typical cost ranges
Here is what we typically see for extensions in our area:
Single-storey rear extension (standard spec): £35,000 to £50,000
Single-storey rear extension (higher spec): £50,000 to £70,000+
Side return / wraparound extension: £45,000 to £75,000+
Two-storey extension: £60,000 to £120,000+
What is included in the price?
Our quotes include all structural work, building regulations compliance, plumbing, electrics, plastering and project management. We provide a fixed price after the survey so you know exactly what you are paying before work starts.
What is usually extra: kitchen units and appliances, floor finishes, decoration and any landscaping outside the extension footprint.
How to plan your budget
Start with a 10 to 15 percent contingency on top of the quoted price. Most well-planned extensions come in within the original budget — but the contingency is there if you need it.
We offer a free initial consultation where we discuss your plans, check what is feasible and give you a realistic budget range before you commit to a full survey.