Calculate Your Side Return Extension Cost
What Is a Side Return Extension?
A side return extension infills the narrow alleyway running alongside a terraced or semi-detached Victorian or Edwardian property. It is most common in London and other cities with large stocks of terraced housing. The result is a wider ground-floor space, most commonly used to create a large open-plan kitchen-diner.
Typical side returns are 2–3 m wide and 4–8 m long, giving a floor area of 8–24 m². The extension is usually single storey with a flat or sloped roof incorporating skylights to maintain natural light in the widened space.
Frequently Asked Questions
How much does a side return extension cost in the UK in 2026?
A side return extension costs £1,800–£3,000 per m² in 2026. A typical 18 m² (3 m × 6 m) project costs £32,000–£54,000 for the build. Add skylights, structural steel and professional fees and most projects land at £40,000–£70,000 total.
Do I need planning permission for a side return extension?
Most side return extensions qualify as Permitted Development in England if they are under 4 m high, do not exceed half the width of the original house, and do not project beyond the front elevation facing a highway. Conservation areas, listed buildings and properties in Article 4 Directions areas require full planning permission.
Why is structural steel needed in a side return extension?
When the original wall between the house and side return is removed to create a wider open-plan space, a steel RSJ (rolled steel joist) is needed to carry the roof load. This typically costs £3,000–£6,000, including the structural engineer's design calculations. Budget for it as an essential cost in most side return projects.
How much do roof skylights cost for a side return extension?
Roof skylights cost £1,500–£2,500 each supply and installed in 2026. Most side return extensions include 1–3 skylights to replace the natural light lost by infilling the side alleyway. Fixed rooflights are at the lower end; opening or electric roof lanterns cost more.
Is a side return extension cheaper than moving house?
Yes, significantly. Moving to a larger home in London costs £30,000–£60,000+ in stamp duty, estate agent fees, survey costs and removals alone — before the price premium of a bigger property. A side return extension at £40,000–£70,000 delivers comparable additional space, no stamp duty liability and no need to uproot your family.