How Much Does a Double-Storey Extension Cost?

A double-storey (two-storey) extension is the more space-efficient way to add a meaningful amount of floor area to a house — and while the total cost is higher than a single-storey extension, it isn't simply double. The economics genuinely work in your favour once you understand why. As a rough starting point, a typical two-storey extension lands somewhere between £45,000 and £150,000+, depending heavily on footprint and specification.

Why two storeys costs less than double

A double-storey extension shares one roof and one set of foundations across two floors of new space, rather than needing a separate roof and foundation for each. Foundation and roof costs are a significant, largely fixed portion of any extension's budget — spreading them across double the floor area is what makes the second storey proportionally cheaper than the first. The widely used rule of thumb in the UK building trade is that a double-storey extension costs roughly 50–60% more in total than a single-storey extension of the same footprint — not 100% more.

Typical UK double-storey extension costs

These are broad, genuinely typical ranges — actual cost depends heavily on footprint, specification, roof design, and ground conditions.

• Combined new floor area cost: roughly £2,200–£3,400 per square metre of total new space across both floors
• Smaller two-storey extension (single room per floor): roughly £45,000–£80,000
• Larger two-storey extension (multiple rooms per floor): roughly £80,000–£150,000+

For comparison, see our single-storey extension cost guide for the equivalent one-floor figures.

A worked example

Take a 4m x 4m (16m² per floor, 32m² total) rear extension — a kitchen extension downstairs, a bedroom or home office above. At roughly £2,200–£3,400/m² for the combined floor area, that's approximately £70,000–£110,000 for the shell and fit-out together, before accounting for any staircase alterations or particularly demanding roof matching. The equivalent single-storey-only version (just the ground floor, 16m²) would typically cost £30,000–£55,000 on its own — illustrating the "second floor for roughly half the marginal cost" economics directly.

What drives the cost beyond the shared-structure saving

Structural complexity upstairs

A second storey needs its own structural support — beams, joists, and often a more substantial foundation than a single-storey build alone would require, since the foundation is now carrying more weight.

Roof design

Matching an existing roofline, or working around an awkward junction between old and new roof structure, is one of the more complex and cost-sensitive parts of a double-storey build.

Staircase access

Getting from the existing house to the new upstairs space usually means altering or extending the existing staircase, or adding a new one — genuine additional structural and joinery work. See our staircase renovation cost guide for a sense of that cost on its own.

What each floor is used for

A ground-floor kitchen extension with a bedroom or bathroom above involves plumbing runs on both levels, which adds cost compared with two straightforward reception rooms.

Planning permission and Permitted Development

Two-storey extensions have tighter Permitted Development limits than single-storey ones. As a general guide (confirm current specifics with your local planning authority): a two-storey rear extension is typically limited to around 3m in depth and generally must not come within 7m of the rear boundary. Side extensions and anything near a boundary are more likely to need a full planning application than an equivalent single-storey project. See our full guide on Permitted Development rights for extensions for the complete picture.

Party wall matters — more likely to apply here

Because a two-storey extension usually involves a taller shared wall with a neighbouring property, and often deeper foundations that can fall within 3m of a neighbour's structure, Party Wall Act notices are more commonly required on double-storey projects than single-storey ones. See our guide on Party Wall Agreements — build the notice period (one to two months) into your programme early rather than discovering it's needed once a design is finalised.

Labour and timeline

See Extension Labour Costs Explained for how the trade cost breaks down, and our common extension mistakes guide for where two-storey extension budgets typically go wrong — structural sequencing errors are more consequential on a two-storey build than a single-storey one, since work at ground level directly supports what's happening above it.

Extension vs moving

A double-storey extension is often the point where the "extend or move" question becomes most relevant, given the scale of investment involved — see our full comparison: Extension vs Moving House.

Getting an accurate quote

Because structural design varies so much between two-storey projects, the only reliable way to get an accurate figure is a proper site visit, structural input, and a detailed itemised quote — a per-square-metre estimate alone can't account for your specific roof junction, foundation needs, or staircase arrangement.