A side return extension can be a really cost-effective way of creating more space and adding value to your property without eating into your precious garden space. It can transform a narrow kitchen into the most amazing living / dining area – even becoming the social hub of your family home! But how much does a side return extension cost?

Cost per square meter

We know how important it is to have a rough estimate on the cost at a very early stage. The easy answer for us is to say that it depends on ‘a number of factors’, which of course it does: the design, materials, the existing site conditions, where you are in the country etc. can ALL affect the cost dramatically. So, the averages below should be taken with a big pinch of salt and the best way of estimating your project cost is to get professional advice from an architect and/or QS who can put together a budget estimate for your particular site and brief.

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We asked our community of architects on Design for Me for their experience. *Figures have been updated in 2022 following recent price increases in constructions costs of approximately 30%, according to a survey of architects and designers on Design for Me.

There are certain economies of scale when it comes to doing building work, so a small extension will cost more per square metre than a larger rear extension. Let’s assume the footprint of the extension is ten square metres for our example:


Side return extension costs per square metre:

Note: there would be additional costs for internal renovation work elsewhere and/or adding in a new kitchen.

In London, a side return extension can cost anywhere between £3,900- £6,500* per square metre (+VAT and professional fees).

Outside London, the cost can vary between £2,600 – £5,200* depending on where you are in the country (+VAT and professional fees).


Case study: A side return extension in Camden

We hope this is a useful starting point for you, but it’s really best to obtain costs on a project-by-project basis. Simon and Jon, of Yard Architects – who have a profile on Design for Me – have kindly shared an example of one of their side return extension projects with us. The budget was a primary concern for the client, which Simon and Jon took on board at each stage of the design process, and we think the result is an absolute architectural gem! See their profile here and shortlist them for your project.


side extension

Brief project description

This is a small side return extension to a Victorian terraced house in the heart of Camden Town. With space at a premium, we designed an inside-out lightweight exposed timber frame construction that creates a working wall incorporating services, shelving and recesses between the structure in the clients new dining area.

A large pivot door opens out into the garden, working alongside the glazed roof to provide a light and airy addition.

Project size (floor area)

Extension was 10 sqm

Construction cost

£42,000 ex VAT and fees (note that this project is now a few years old, so the cost would likely be higher now)

Level of finish


Project location

Camden, London


side extension


Any project specific features that increased costs?

It was decided to spend more to find a really wide door to lead into the garden; we wanted something extra wide so the view was not interrupted. We had to use a commercial door supplier in order to get the extra width. A nearby tree meant that some of the foundations had to be much deeper than anticipated.

Any particular cost saving strategies used in the design? 

The whole concept of the extension was based on reducing the construction cost and paring down the design as far as we could. We built the new ‘working wall’ inside out, with the structure left exposed internally. Normally you would put insulation between the timber studs and plasterboard on top. This gained a bit of space and allowed us to design in functional and practical shelves for storage. It also meant less trades involved, saving costs and time. We used standard building materials and left as much of the existing house intact as we could, such as the exposed brick wall to the side of the house.


side extension

side extension

side extension



Controlling your budget

Getting professional advice is the best way to manage your budget. A good architect will work with you to make sure you are spending your budget in the right way and advise where costs would be unnecessary or could be minimised. For larger or more ambitious extensions, you may benefit from using a quantity surveyor too.

Make sure you get three to five quotes from recommended builders (your architect can help compile a tender list for you) to get a firm idea of the costs before starting work.


Finding the right architect

Choosing the right architects for your home project is a critical first step, but can also be a bit of a minefield. This is exactly why I set up Design for Me in the first place. All you need to do is tell us a bit about your extension project and we’ll help you find the right architect or architectural designer for free!


Emily  Design for Me

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