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How Much Does a UK House Extension Cost in 2024

Updated: Apr 20

I've run my own architecture practice, designing house extensions since 2009. When I started, decent house extensions could be built for less than £1500 per square meter. I recently built a house extension in Edinburgh and I've posted the full cost breakdown near the end of this blog post.

how much does a house extension cost in the UK 2024
House extension costs per square meter

The project is so recent, I don't yet have finished photos but I can tell you how much it cost.

Before I do though, lets have some context.

First off the existing house is on a steep site, and this usually makes building more expensive.

house extension costs in the UK can increase if the site is steeply sloping
steep sites make house extensions more expensive

Just look at that slope. Aside from the effort of moving materials up and down that slope, the building design itself had to take the change of level into account.

The new design, in yellow, had to step down the hill. If we didn't do this, the entrance at the driveway would be far too high. As it was, we still had to design in a set of stairs. That said, the changes of level inside the new extension created the opportunity to make a really great open plan living space.

a house extension under construction
A split level open plan house extension, under construction

This photo shows what I mean, there are three levels from left to right; the kitchen (on the same level as the existing house) then down two steps to the dining space, then down two more steps to the living space.

Another thing that usually costs money, but was hard to avoid in this case, was the shape of the extension.

A house extension floor plan
Floor plan showing new work highlighted in yellow

The geometry of this house extension is not orthogonal. To keep costs to a minimum always build at 90 degrees. As they say, if it ain't a right angle, its a wrong angle.

The site had some odd geometry and in order to maximise space we built up to the boundary with the neighbours (thats the thick, dashed red line on the floor plan) This decisions meant we needed to use lots of steel to support the roof in the open plan space and that cost us.

So the design of this house extension was forced down a route that increased the budget.

Split level cost more but we could not avoid it. Building using odd angles cost more but we needed the space. However, we saved money by making two other big design decisions;

First, as little work inside the existing house as possible.

Second, build a flat roof on the house extension.

A house extension being built in the UK
House extension under construction

At the back of the photo above is an existing door in the side of the house. We re-used that door and, crucially, did as little work inside the house as possible. This kept costs for the extension under control.

Floor plan of a house extension showing downtakings
Floor plan showing demolition in thin, dashed red lines

The thin, dashed red line on the floor plan above show the existing parts of the house and external landscaping that we demolished. Inside the existing house we stripped out the kitchen units and converted the room into a utility. The new extension would house a larger kitchen.

House extension with a flat roof saves money
A flat roof house extension costs less to build

Say what you like about flat roofs, they usually save money on house extensions because they limit the amount of work required to connect with the existing roof. Using a conventional, pitch roof would require more material to build and more time to connect with the existing house.

So what did it cost to build this house extension?

Time for the big reveal. This is the cost breakdown of the main elements of the extension.

How much a house extension costs in 2024 in the UK
Breakdown of construction costs for a UK house extension in 2024

The extension has a gross internal floor area of 63 square meters, the overall cost works out at £3523 + VAT per square meter. In 2024 that is actually pretty good. Considering the design challenges we faced with the existing site, the end result is value for money.

If the site had been flat, didn't require a split level open plan and could be built as a square box, it might have been possible to build it for £3,000 + VAT per square meter. That is the baseline figure I quote to clients across the UK during online consultations. It is possible to build house extensions for much more than £3,000 + VAT per square meter. Ive built a few for more than twice that sum in recent times.

If you are planning to build a house extension anywhere in the UK, and you would like advice from an experienced architect, get in touch and book a consultation with me to discuss your ideas, timescale and budget. I provide a sense check on your ideas and can advise you on how to find the right local architect for your project.

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