Cost Per Square Metre by Extension Type
| Extension type | Typical size | Total cost | Cost per m² |
|---|---|---|---|
| Single storey rear | 15–20m² | £28,000–£50,000 | ~£1,400–£2,500 |
| Side return / wrap-around | 20–25m² | £45,000–£75,000 | ~£1,800–£3,000 |
| Double storey | 30–35m² combined | £55,000–£100,000+ | ~£1,550–£2,850 |
| Orangery / garden room | 15–18m² | £32,000+ | ~£1,780+ |
Why Double Storey Often Costs Less Per Square Metre
It seems counterintuitive, but a double storey extension is frequently the best value per square metre of all extension types. The reason is simple: the most expensive elements of any extension, groundworks, foundations, and the roof, are built once and shared across two floors of new space instead of one. You get a larger kitchen-diner downstairs and an additional bedroom (often with en suite) above, for a total cost that's lower per square metre than building the same downstairs space alone.
This is why double storey extensions are particularly popular on 1930s semis in Sale, Altrincham and Stretford, where families need both extra reception space and an additional bedroom, building both at once is more cost-effective than two separate single storey projects over time.
Why a Side Return or Wrap-Around Costs More Per Square Metre
Side return extensions, infilling the narrow side passage alongside a Victorian or Edwardian terrace, and wrap-around extensions (combining a side return with a rear extension) tend to sit at the higher end per square metre. The narrow, often irregular shape of a side return means more linear metres of new wall and foundation relative to the floor area gained, which pushes up the cost per square metre even though the total project cost may be similar to a larger single storey rear extension.
What Cost-Per-Square-Metre Figures Don't Tell You
A per-m² figure is useful for comparing extension types at a glance, but it can be misleading if you rely on it alone. Several factors shift the real cost without changing the floor area:
- Glazing specification — bifold or sliding doors across a wide opening cost significantly more than a single set of French doors
- Roof type — a flat roof with rooflights costs less than a vaulted or lantern roof structure
- Site access — terraced houses with no side access often need materials carried through the property, adding labour time
- Below-ground conditions — older Manchester properties sometimes have unexpected drainage runs or poor ground conditions that affect foundation depth and cost
- Kitchen fit-out — many rear extensions include a new kitchen, which is priced separately from the structural shell
A Genuinely Useful Comparison: Extending vs Moving
Even at the higher end of these per-m² ranges, extending is consistently cheaper than moving to a larger property once stamp duty, estate agent fees and the price difference between properties are factored in — often by £80,000–£150,000+ in Greater Manchester's current market. See the full comparison on our house extension service page.
How to Get a Real Number, Not Just a Per-m² Estimate
Per-square-metre figures are a planning tool, not a quote. The only way to know what your specific extension will cost is a site survey, where the structural complexity, ground conditions, and your chosen specification can be properly assessed. We provide a free site survey and return a fixed, itemised quote, not an estimate based on floor area alone. Browse completed extensions, including before-and-after detail, on our projects page.