Side return extensions: a structural guide for terraced houses

What structural work does a side return extension need?

A side return extension needs a structural engineer to design three main elements: a long steel beam to replace the side wall of the back addition, a new wall and foundation along the boundary, and the roof that spans between them. The foundations must also suit the ground, which across much of London means shrinkable clay. Building Control will want calculations and drawings for all of it before approving the work.

The side return is the narrow alley, often a metre or two wide, between your back addition (the narrower rear wing, sometimes called the outrigger) and the boundary with next door. Infilling it turns a galley kitchen into a wide room. Many projects also wrap round the back. The general points about extending to the rear are in our rear extension structural guide, so this page focuses on what is particular to the side.

Taking out the side wall of the back addition

The flank wall of the back addition typically carries the first floor walls and floor above it, often a bathroom, and in some houses a chimney breast that once served a kitchen range. Removing it at ground floor means a steel beam has to pick up all of that, along most of the length of the room.

These beams are long, so stiffness matters as much as strength. A beam that is strong enough can still sag enough to crack the plaster and tiling above. The engineer sizes it to limit deflection, and may use a deeper section, a pair of beams or an intermediate post if the span and loads call for it.

The ends of the beam need proper support. At the house end it often bears onto the main rear wall or a steel post. At the garden end it may sit on a post with its own pad foundation. Old soft brickwork under a bearing may need replacing with a concrete padstone or engineering bricks, and any chimney breast above has to be supported or removed as part of the design.

The boundary wall: build new or reuse what is there?

Most side returns run along a boundary. The existing garden wall is rarely suitable as the side wall of a habitable room. It is often a single brick thick, built on shallow or no footings, and may be leaning, cracked or jointly owned with the neighbour. The usual options are:

Building a new wall on the line of the boundary, or excavating close to and below a neighbour's foundations, brings in the Party Wall etc. Act 1996 and its notice periods. Where the boundary is formed by the neighbour's own back addition wall, the new roof may bear against it, which also needs notice and careful detailing. Our guide to party walls and structural works explains the notices.

Height near the boundary is usually limited by planning, which is why many side returns have a low eaves on the neighbour's side with the roof rising towards the house. That shape affects where loads land, so the structural design follows the agreed planning drawings.

• A new wall built wholly on your side of the boundary, with its own foundation
• A new wall built astride the boundary as a party wall, with the neighbour's agreement
• Building up on the existing wall, only where investigation shows it can take the load, which is uncommon

Foundations in a narrow space

Side returns are tight sites. The new foundation runs right beside your own back addition and often the neighbour's too, and Victorian footings are typically shallow, sometimes just a few courses of stepped brickwork. Digging deeper alongside them can undermine them, so excavation may need to be done in short sections, with local underpinning where the new and old foundations meet.

On London Clay, the required depth is driven by the clay's shrinkage and by trees, including those in neighbouring gardens. A trial pit at the start shows how deep the existing footings go and what the ground is like. Where a new deep foundation meets an old shallow one, the detail must allow for slightly different movement, so a crack does not open along the junction.

Roofs, glazing and stability

Side return roofs are commonly pitched glazing, a flat roof with long rooflights, or a combination. The roof usually spans from the new boundary wall to the beam carrying the back addition, so its load is added to that beam. Frameless or structural glazing needs tight deflection limits on the steel around it, or the glass can crack or leak.

Large areas of glass and an open plan reduce the amount of solid wall bracing the back of the house. In a wraparound, the corner where the side and rear extensions meet often becomes a single steel post carrying two beams. Where the rear is almost entirely glazed, that post may need to form part of a rigid frame, or a section of wall may need to be retained, to keep the building stable.

Drains, soil stacks and hidden items

The side return is where many terraces route the soil stack, rainwater pipes and a gully, often draining into a shared run along the backs of the terrace that is now likely to be a public sewer. Moving the stack, rerouting a drain or building over it needs planning before the layout is fixed. A CCTV survey early on shows where each pipe goes, and our guide to building over a public sewer covers the approval needed if a shared drain is involved.

Other common finds include an old coal store or outside WC, a manhole in the middle of the alley, made ground from past works and a boundary wall that turns out to be the neighbour's. None of these is unusual, but each is cheaper to deal with in the design than during excavation.

Getting the design done

Our extension structural package covers the beams, posts, foundations and roof for a side return or wraparound, with calculations and drawings ready for Building Control. It starts from £850 + VAT, as a fixed fee confirmed in writing before work starts. Every design goes through a three-step quality control process, with final sign-off by our Head of Engineering. Side returns are a familiar job for us in the Victorian terraces of boroughs such as Hackney.