Side Return Extension: Planning, Costs, and Design Ideas

A complete guide to side return extensions in the UK, covering planning rules, permitted development, costs, and how to make the most of the narrow space beside your home.

A side return extension - sometimes called an infill extension - fills in the narrow alleyway that runs down the side of many terraced and semi-detached houses. It is one of the most popular home improvements in the UK, particularly in London and other cities where every square metre counts.

What Is a Side Return?

The "side return" is the gap between the side wall of your house and the boundary with your neighbour. In Victorian and Edwardian terraced houses, this is typically the space beside the kitchen, between the main house and the original rear addition. It is usually 0.9 to 1.5 metres wide.

A side return extension fills in this gap, creating a wider, open-plan kitchen-diner or living space. It is usually single storey with a flat roof or a glass roof, and connects to a rear extension if one is being built at the same time.

Do You Need Planning Permission?

Permitted development

A side return extension often falls within permitted development rights, meaning no planning application is needed. The key rules are:

• The extension must be single storey
• Maximum height: 4 metres
• If within 2 metres of a boundary: maximum eaves height of 3 metres
• It must not extend beyond the rear wall of the original house by more than 3 metres (or 4 metres for detached houses) under standard PD
• It must not cover more than 50% of the garden
• Materials must be similar in appearance to the existing house

Since a side return typically extends alongside the existing kitchen rather than projecting beyond the rear wall, it often fits within these limits.

When you need planning permission

• In a conservation area - side extensions are restricted under PD in designated areas
• If your PD rights have been removed by an Article 4 direction
• If you live in a flat - PD does not apply
• If the extension exceeds PD limits (e.g. two storey, or combined with a large rear extension)

Typical Costs

Side return extensions vary in cost depending on size, specification, and location. As a rough guide for 2026:

These costs include the building work, roof, flooring, electrics, and plumbing but exclude kitchen units and appliances.

Design Ideas

Glass roof

The most popular option for side returns is a glass or glazed roof. Because the space is narrow and between two walls, a solid roof can make it feel dark. A glass roof floods the new space with natural light and makes the kitchen feel dramatically larger.

Bi-fold or sliding doors

If the side return connects to the garden at the rear, bi-fold or sliding doors create a seamless indoor-outdoor flow. This works particularly well with an open-plan kitchen-diner layout.

Rooflight strip

If a full glass roof is too expensive or causes overheating concerns, a single rooflight strip running the length of the extension is a good compromise. It provides excellent natural light at a lower cost.

Party Wall

If you are building on or near the boundary with your neighbour, you will need to serve a Party Wall Notice. For a side return extension, this is almost always required because the new wall typically sits on or very close to the boundary line.

Budget around £700-1,500 per neighbour for party wall surveyor fees.

Building Regulations

All side return extensions require building regulations approval, covering structural calculations, drainage, insulation, ventilation, and electrics. Your builder or architect will typically handle this through a building control application.