Which Extension Type Costs Less? (Compare 2026 UK)
If your goal is more space for the lowest cost, the type of extension you choose matters more than almost any other decision. In 2026 a single-storey rear extension is the cheapest true extension per square metre at £2,000-£3,000/m², while a double-storey extension is cheaper per m² but costs more overall. Conservatories and garden rooms cost the least in absolute terms but add different kinds of space. A wraparound is the most expensive per project. This guide compares every extension type on cost so you can extend for less.
Extension Types Ranked by Cost
Cost per square metre and total project cost tell two different stories. A double-storey extension is cheaper per m² because you share one foundation and roof across two floors - but the bigger project costs more in total. Pick the type that matches your budget AND the space you actually need.
Single-storey rear (cheapest true extension)
£2,000-£3,000/m². The default value option - one foundation, one roof, straightforward build. A 20m² rear extension typically runs £40,000-£60,000 including finishes.
Side-return infill
£2,200-£3,200/m². Fills the alley beside a terrace or semi. Small footprint but high impact on a kitchen-diner. Often £30,000-£50,000.
Double-storey (cheapest per m²)
£1,800-£2,600/m². Adding a second floor shares foundations and roof, so each m² is cheaper - but total cost is higher (£70,000-£120,000) because you build more.
Conservatory (least overall)
£10,000-£30,000 total. Cheapest way to add a bright room, but performs poorly thermally unless upgraded. Best as a garden room rather than year-round living space.
Garden room / modular
£15,000-£40,000 total. A standalone insulated room in the garden. No party-wall issues, fast build, but detached from the house.
Wraparound (most expensive)
£2,200-£3,200/m² but the largest footprint, so total cost is highest at £60,000-£100,000+. Combines rear and side-return for a transformed ground floor.
Costs are indicative 2026 UK ranges including construction and standard finishes; groundworks, glazing and location can move figures significantly. Always compare like-for-like quotes.
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