Side return extensions – How to build and how much it'll cost

Side return extensions – How to build and how much it'll cost

For many homeowners, the side-return area – the pathway that runs alongside the ground floor area of the house – is a redundant space. But by building an extension on to it, you can make your home bigger and better.

Experts at Resi explain, ‘If you have alleyway space to the side of your property and are looking to create some extra room, a side return extension might just be what you’re looking for. Side returns (sometimes referred to as side infills) allow you to expand out into an alleyway, turning an unused area into much desired living space.’

Image credit: Veronica Rodriguez

‘Side returns are often applied to Victorian terraced properties as a lot of them possess alleyways in their configuration. By filling in this side alleyway, L shaped properties are squared off, creating that all important additional space.’

‘The perks of a side return extension compared to other common extensions, like a rear extension, is that it tends to use up ‘dead space’, rather than the best parts of your garden. Ideal for anyone living in areas where green space is at a premium.’

So we’ve got the lowdown on everything you need to know about building a side extension.

How much will a side extension cost?

A simple side extension design will cost anything from £20,000 to £50,000 dependent on the area you live, how large an extension you’d like, the design complexity, which contractors you use and the building materials you want.

Extra internal work, such as adding a new kitchen, and the style of the extension – for example adding glazed external doors – will affect the final figure.

What structural work is involved?

  1. A new wall is built on either the boundary of you and your neighbour’s land, or solely on your side of the boundary.
  2. A roof is added (consider a fully glazed design or one with several skylights to flood the interior with light).
  3. The side wall to the existing rear room is either completely or partially knocked through. A steel frame may need to be inserted in the wall to support this new opening.
  4. A new floor is usually put in, level with the existing floor.
A side return extension with kitchen and dining are looking over the garden

Image credit: David Giles

Will I need an architect?

Not necessarily. Some conservatory companies and specialist builders offer a complete ‘design and build’ package service, which saves paying extra fees to architects or structural engineers for drawing up plans.

Do I need planning permission?

Council rules changed in October 2008, so you no longer need planning permission for a ground floor side-extension under permitted development as long as the extension is only single storey, no more than 4m high and no wider than half the width of the original house.

You’ll still have to comply with Building Regulations and possibly with the Party Wall Act if you’re creating a new boundary wall between you and your neighbour, so speak to your local council’s planning office for details.


Image credit: Alasdair MacIntosh

How long will it take?

Plan for at least eight to ten weeks for the structural work, plus more for decorating the interior of the new space.

Who do I contact?

The Federation of Master Builders, 020 7242 7583,, has details of registered tradesmen.

Local Authority Building Control, 020 7641 8737,, can advise on applying for Building Regulations Approval.

The Institution of Structural Engineers, 020 7235 4535,, has details of engineers.

Royal Institute of British Architects, 020 7580 5533,, has a list of architects in your area.

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