Single-Storey vs Double-Storey Extension: Cost Breakdown (2026)

The question sounds like it should have an obvious answer — surely two floors cost twice as much as one? They don't. A double-storey extension is almost always the better value per square metre, because the expensive bits (foundations, the roof, scaffolding, getting a crew to site) are shared across two floors instead of one.

The 2026 numbers side by side

	Single-storey	Double-storey
Build cost per m²	£1,500–£2,500	£1,800–£2,800
Floor area added	~20 m²	~40 m²
Typical total	£30,000–£55,000	£50,000–£95,000
Cost per m² of space	higher	lower

Double the floor area for roughly 60–75% more money, not 100% more. If you have any thought of needing the upstairs space (a bedroom, an en-suite, a study), building it at the same time as the ground floor is far cheaper than coming back in five years to do it separately.

When single-storey is the right call

• You only need ground-floor space — a bigger kitchen-diner or a family room.
• Permitted Development matters to you: single-storey rear extensions often fall under PD rights, while double-storey usually needs full planning permission.
• Upstairs there's nothing to gain — the extension would sit under an existing window or over a tricky roof.
• Budget is firmly capped and the upper floor isn't worth borrowing more for.

When double-storey wins

• You need bedrooms or bathrooms as well as living space.
• You plan to stay long term and want to do the disruption once.
• You're optimising resale value — an extra double bedroom and bathroom usually returns more than the same spend on ground floor alone.

The hidden cost that catches people out

Double-storey work is more likely to trigger a Party Wall agreement with neighbours, full planning permission rather than Permitted Development, and more involved structural engineering. None of these are deal-breakers, but they add weeks and a few thousand pounds. Factor them in from the start rather than discovering them after you've fallen in love with the drawings.

FAQ

Is a double-storey extension twice the price of single-storey?

No. Expect roughly 60–75% more than an equivalent single-storey, for double the floor area — which is why the cost per usable square metre is lower.

Do I need planning permission for a double-storey extension?

Often yes. Many single-storey rear extensions fall under Permitted Development, but double-storey extensions more commonly need a full householder planning application. See our planning permission guide.

Related: How much does a house extension cost in the UK? · Loft conversion vs extension: which is cheaper?