Is a side return infill worth it on a narrow terrace?

A side return is the narrow strip of outside space running alongside the back addition of a Victorian or Edwardian terrace. It is usually somewhere between one and two metres wide, often paved, and almost always used for bins, a bike and a drain cover.

Filling it in adds less floor area than any other extension. It also changes how the ground floor feels more than almost anything else you can do to a terrace.

What the strip is actually doing to your house

The original back addition makes the ground floor an L shape. The kitchen ends up long and narrow, with one wall of windows facing a wall of brick a metre away, and a dining table somewhere it does not quite fit.

Infilling the side return squares that L off into a rectangle. The kitchen runs the full width of the house rather than the width of the back addition, which is often the difference between a galley you pass through and a room you can put a table in the middle of.

The light problem, and how it gets solved

The obvious worry is that filling in the gap removes the windows that lit the kitchen. It does. The reason it works anyway is that the new roof is where the light comes from instead.

A side return roof is usually flat or shallow, so rooflights over the new strip bring light down from above, into the middle of the plan rather than the edge of it. Overhead light travels further into a room than a side window does, which is why a squared-off kitchen with rooflights usually ends up brighter than the narrow one it replaced.

This is also why the glazing choice matters so much here. It is doing the job the old windows used to do.

What it costs, and why per square metre is the wrong measure

A side return infill is usually quoted somewhere between £45k and £85k, which is less in total than a full rear extension but more per square metre.

That is not a surprise once you know where extension money goes. Foundations, a structural opening, a roof and Building Control cost roughly what they cost whether the new floor is ten square metres or twenty five. A side return buys a small amount of floor and still pays all of the one-off costs, which our guide on what a rear extension really costs goes through in detail.

So judge it on what the ground floor becomes, not on pounds per square metre. By that measure it is frequently the best value work we do.

When it is usually worth it

The case is strongest when the strip is genuinely dead space, the kitchen is the room that frustrates you, and the garden is short enough that you do not want to give any of it up. A side return takes none of the garden, which on a small London plot is often the deciding factor.

It also suits people who want one build rather than two. If you are likely to want more space later, combining the side return with a rear extension as a wrap-around means one set of foundations, one roof and one period of disruption.

When it is not

If what you actually need is a bedroom, this is the wrong project. A side return adds ground floor and nothing else. A loft conversion or a two-storey extension is the answer to a bedroom problem, and our guide on whether to build up or build out covers how to choose.

There are two other reasons to pause. In a conservation area the infill may need planning permission rather than falling under permitted development. And the party wall is unavoidable here: you are building right up to the boundary, alongside a neighbour, usually digging foundations within a few metres of their wall. Notice has to be served, which our guide to party wall notices explains.

Neither is a reason not to do it. Both are reasons to find out where you stand before you spend money on drawings. If you want someone to look at the strip beside your back addition and tell you honestly what it would give you, book a survey.