The Short Answer
On our estimates, filling in the side return of a terrace or semi costs about £2,600 to £4,200 per m² including VAT, more per m² than an ordinary rear extension (£2,400 to £3,200). A typical 10m² side return is about £30,100 to £53,800 with drawings, an engineer and building control.
I think it costs more per m² because the space is narrow, there's usually a glazed roof, a long steel beam where the side wall comes out, and a neighbour's wall on the boundary. Budget £1,000 to £3,000 per party wall surveyor unless your neighbour consents in writing.
Extension cost calculator
The build priced per square metre, plus the fees most quotes leave out: design, engineer, building control, planning and party wall.
Estimated total
£34,210 to £62,480
| Side return extension, 10m² of floor, standard | £26,000 to £42,000 |
| Design: drawings for planning and building regulations | £3,000 to £9,000 |
| Structural engineer | £300 to £1,200 |
| Building control | £800 to £1,600 |
| Party wall surveyor (one neighbour) | £1,000 to £3,000 |
| Contingency (10%) | £3,110 to £5,680 |
Side return extension prices
| Floor area | Build | With fees |
|---|---|---|
| 6m² | £15,600 to £25,200 | £19,700 to £37,000 |
| 8m² | £20,800 to £33,600 | £24,900 to £45,400 |
| 10m² | £26,000 to £42,000 | £30,100 to £53,800 |
| 12m² | £31,200 to £50,400 | £35,300 to £62,200 |
Party wall notices
A side return almost always involves the Party Wall Act. You must serve notice:
- at least 1 month before building a new wall on the boundary line;
- at least 2 months before cutting into or raising an existing party wall (for a beam bearing, say);
- at least 1 month before digging within 3m of their building and deeper than its foundations (or within 6m if it goes deep enough to cut a 45-degree line down from their foundations).
If the neighbour consents in writing, that's it. If they don't consent within 14 days, the Act treats it as a dispute: you agree on one surveyor, or each appoint one and they pick a third. Budget £1,000 to £3,000 per surveyor on our estimates. I'd talk to the neighbour before the notice arrives: it's the cheapest thing you'll do.
Planning and the glazed roof
A side extension is permitted development on a house in England if it's single storey, no more than 4m high, no wider than half the original house, and, within 2m of the boundary, has eaves no higher than 3m. It isn't permitted development at all in conservation areas and other designated land. Otherwise it's a householder application (£548).
A glazed roof is what makes a side return feel bright, but it counts towards the building regulations glazing limit (25% of the floor area, plus what the extension covers). Over that, you need an energy calculation; I'd expect many glazed side returns to need one, and to solve it with better glass or a solid section of roof. A lantern is £2,200 to £4,500; a full glazed roof costs much more.
Planning a side return and rear together (a wraparound)? Price it as a house extension.
Other prices you may see
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Frequently asked questions
How much does a side return extension cost?
On our estimates, about £2,600 to £4,200 per m² including VAT, so £30,100 to £53,800 for 10m² with drawings, an engineer and building control.
Do I need planning permission for a side return extension?
Not if it's single storey, up to 4m high and no wider than half the original house, and you're not in a conservation area or other designated land. Otherwise yes.
Do I need a party wall agreement for a side return?
Almost always you need to serve a party wall notice, because you're building on or near the boundary or digging near the neighbour's foundations. If they consent in writing, no surveyor is needed.
Can you do a side return extension for 50k?
Often, for a small one: 10m² is about £30,100 to £53,800 with fees on our estimates, before party wall costs and contingency. £50,000 gets tight with a big glazed roof.
Why does a side return cost more per m² than a rear extension?
In my view, because it's a small, narrow space with a long steel beam, a glazed roof and a boundary wall, so the fixed costs are spread over fewer square metres.
How we priced thisView sources (7, 5 official)
Rules come from the legislation and official guidance, read at source. Prices are our estimates: we check them against trade price data, retailer shelf prices and specialists' price lists, then build them up from their parts so the totals add up. Below are the sources behind the page; the trade price guides we only compare against aren’t listed. Where sources disagreed, we've said so. More on our method.
- OfficialThe Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2 Part 1, legislation.gov.uk. Checked 16 Oct 2026.
- OfficialParty Wall etc. Act 1996, sections 2 and 3, legislation.gov.uk. Checked 15 Oct 2026.
- OfficialParty Wall etc. Act 1996, sections 5, 10 and 11 (disputes, surveyors, expenses), legislation.gov.uk. Checked 15 Oct 2026.
- OfficialApproved Document L: Conservation of fuel and power, Volume 1 Dwellings (2021 edition incorporating 2023 amendments), GOV.UK. Checked 16 Oct 2026.
- OfficialA guide to fees for planning applications in England (from 1 April 2026), Planning Portal. Checked 15 Oct 2026.
- Trade guideWhat does a side return extension cost?, Checkatrade. Checked 15 Oct 2026.
- Trade guideHouse extension cost breakdown, Checkatrade. Checked 15 Oct 2026.
