How much does a side return extension cost?

A side return extension costs about £26,600 to £43,400 including VAT in 2026 (£22,199 to £36,162 before VAT) for 8 to 19 m², measured outside. A typical one, 6 m along the side of the house and 2.2 m out to the boundary, comes to about £35,109 including VAT (£29,258 before VAT), or about £2,216 a m² before VAT. That's built, plastered, wired and decorated, with the side wall knocked through into the kitchen.

A side return is the narrow strip down the side of a terraced or semi-detached house, beside the back addition. Building over it brings that strip into the kitchen. The new wall usually goes on the boundary, so the roof drains into a box gutter behind a parapet rather than over the neighbour's side.

What's in the price

We priced a 6 × 2.2 m side return on a Victorian terrace: a solid brick side wall to open up, a lean-to roof at 22.5° with three roof windows, and French doors at the back. This is where the money goes, before VAT.

• Foundations and floor: 8.2 m of trench, 600 × 1000 mm, dug by mini digger and filled with C25 ready-mix, £1,906, and the walls up to the DPC, £1,044. Then a 10.8 m² floor with hardcore, DPM, 120 mm PIR, mesh, concrete and a 75 mm screed, £2,537.
• Skips and muck-away: for the trench spoil, the floor dig and the brickwork from the opening, £1,151.
• The knock-through: a 5.5 m opening in the solid side wall, propped, with a steel 5.8 m long on padstones (a UB 356×171×45 in our example: the engineer sizes it), lifted in, and made good, £3,160.
• Walls: 18.6 m² of cavity wall, brick outside (about 1,102 bricks at a £1.77 allowance) and block inside, with 100 mm PIR, £5,676.
• The parapet: 6 m of timber box gutter lined in Code 5 lead, with an outlet and hopper, and aluminium coping on the wall, £1,598.
• The roof: rafters, membrane, battens and concrete interlocking tiles, with lead flashing where it meets the house, £2,884, and 100 mm PIR between the rafters with 50 mm over them, £635.
• Glazing: three Velux MK06 roof windows, trimmed in, flashed and lined, £3,694, and aluminium French doors 1.3 m wide, £1,766.
• Inside: dry lining and skim, the ceiling, skirting and decorating, £2,068, and electrics, £1,140.

That's about 364 hours of work in all. The walls and the glazing are the biggest parts, more than a third of the price between them.

Price by size

• 4.5 × 1.8 m: £22,199 before VAT, £26,639 including VAT
• 6 × 2.2 m: £29,258 before VAT, £35,109 including VAT
• 7.5 × 2.5 m: £36,162 before VAT, £43,395 including VAT

All with a parapet on the boundary, a tiled lean-to with roof windows, French doors and a knock-through the length of the room. Not included are architect and engineer fees, building control, the party wall surveyor, the kitchen, floor coverings and heating.

Pricing it as a builder

Price the side return as two new walls, the boundary wall and the short return across the back, with the foundations under those only. The roof leans against the house wall, so price the flashing along it. Then the parapet and box gutter as their own item, and the knock-through with its steel. On our 6 × 2.2 m job that's about 28 hours a m² of outside footprint.

Check these on the site visit. Whether the boundary wall is yours to build on or a shared wall, which decides the party wall notice and maybe the foundation. Where the drains run, because they often go down the side return. How long the opening is and what sits on the wall above it, so the engineer can size the steel and any posts. And the height of the parapet, since the roof has to drain into the box gutter behind it.

Priced on its own, a job like this carries a small premium for travel, setting up, a minimum charge and part-used materials. Done as an extra while you're already on site, the same work usually costs a bit less.

How we priced it

We built the job in Bricken, the quoting app for UK builders, with its extension starter, item by item. Standard merchant prices for the materials, a £1.77 allowance per brick, typical hourly rates for each trade, and a typical builder's markup of 30% on materials and 35% on labour. Your builder will price it at their own rates, so get a few quotes.

Priced in Bricken at standard UK rates (4.5 × 1.8 m for the low figure, 7.5 × 2.5 m for the high). Each figure is then multiplied by Bricken's regional factor, from 0.90 in the North East to 1.30 in inner London. The factors come from public 2025-26 build-cost guides and our own quotes. Where a region covers two areas, the low uses the lower factor and the high the higher. Treat small differences as a guide.