A double storey extension in the West Midlands costs between £85,000 and £160,000 in 2026, typically £1,800 to £2,600 per square metre of floor area created.
The headline number is higher than a single storey, which puts people off. The per-square-metre number is lower, which is the number that actually matters. You dig the foundations once, build the roof once, and get two floors out of it.
What a double storey extension costs in 2026
West Midlands ranges for a full design-build: drawings, structural engineering, Building Regulations, groundworks, superstructure, roof, glazing and internal finishes.
| Size and type | Typical range | Usually creates |
|---|---|---|
| Small rear, two storey | £85,000 – £110,000 | Kitchen extension below, bedroom above |
| Standard rear, two storey | £110,000 – £140,000 | Kitchen-diner below, bedroom and en-suite above |
| Side and rear, two storey | £140,000 – £180,000 | Larger ground floor plus two rooms above |
| Two storey with structural glazing | £160,000 – £220,000 | As above with feature glazing and higher spec |
Why two storeys beat two singles
The fixed costs of an extension do not double when you add a floor. Excavation, foundations, drainage diversions, scaffolding, the roof structure and the roof covering are largely one-off costs regardless of how many storeys sit between the ground and the roof.
What does scale with the second floor is the walls, the floor structure, the glazing and the internal finishes. That is why the per-square-metre rate typically drops by fifteen to twenty-five per cent compared with a single storey of the same footprint.
The practical implication: if you think you might want more bedrooms within ten years, building up now is materially cheaper than extending again later — and it avoids a second round of disruption, professional fees and scaffolding.
Planning permission is more likely
Single storey rear extensions often fall under permitted development. Two storey extensions frequently do not, and the rules are tighter.
Under permitted development a two storey rear extension is limited to three metres in depth, must be at least seven metres from the rear boundary, cannot be higher than the existing roof, and must not sit on the principal elevation. Side extensions at two storeys are generally excluded from permitted development altogether.
In practice most two storey extensions in Sutton Coldfield, Solihull and Harborne go through a full planning application. Allow eight to twelve weeks for a decision, and factor in that a neighbour objection does not automatically refuse an application but does slow it down.
The Party Wall Act will apply
If you are extending a semi or a terrace, and almost any two storey extension near a boundary, the Party Wall etc. Act 1996 will apply. This is a separate legal process from planning and Building Regulations, and it catches people out because nobody mentions it until late.
You must serve notice on affected neighbours two months before work starts on a party wall, or one month for excavation near their foundations. If they consent in writing, that is the end of it. If they dissent — which is common and not necessarily hostile — surveyors are appointed and a party wall award is drawn up, typically costing £1,000 to £2,000 per neighbour, paid by you.
Serve notice early. A two month notice period discovered at week ten of a twelve week lead-in is the most avoidable delay in this entire process.
Foundations and what sits underneath
A two storey extension carries considerably more load than a single storey, and the foundation design reflects that. Depth is determined by soil type, proximity of trees and what Building Control require after inspecting the trial hole.
Clay soils, which are common across parts of the West Midlands, shrink and swell with moisture. Mature trees nearby can push foundation depth to two and a half metres or more, and each additional metre of depth adds meaningfully to the cost.
This is the biggest single unknown in an extension quote. A builder who has not dug a trial hole is estimating, and a quote based on an estimate of foundation depth is a quote that can move.
How long it takes
Sixteen to twenty-four weeks on site for most two storey extensions. Add eight to sixteen weeks beforehand for design, planning determination, structural calculations and party wall notices.
Realistically, from first conversation to finished room is nine to twelve months. Most of that is not building — it is consent. Starting the planning and party wall processes early is the single biggest lever on the overall timeline.
Frequently asked questions
How much does a double storey extension cost in the West Midlands?
Between £85,000 and £160,000 in 2026, or roughly £1,800 to £2,600 per square metre. A standard two storey rear extension creating a kitchen-diner below and a bedroom with en-suite above typically costs £110,000 to £140,000.
Is a two storey extension better value than a single storey?
Per square metre, yes. Excavation, foundations, drainage, scaffolding and the roof are largely one-off costs regardless of the number of storeys, so adding a second floor typically reduces the per-square-metre rate by fifteen to twenty-five per cent compared with a single storey of the same footprint.
Do I need planning permission for a two storey extension?
Usually yes. Permitted development for two storey rear extensions is limited to three metres depth, at least seven metres from the rear boundary, no higher than the existing roof, and not on the principal elevation. Two storey side extensions are generally excluded entirely. Allow eight to twelve weeks for a decision.
Does the Party Wall Act apply to a two storey extension?
Almost always for a semi or terrace, and usually for any extension near a boundary. You must serve notice two months before work on a party wall, or one month for excavation near a neighbour's foundations. If a neighbour dissents, surveyors are appointed and an award drawn up, typically costing £1,000 to £2,000 per neighbour, paid by you.
How long does a double storey extension take?
Sixteen to twenty-four weeks on site, plus eight to sixteen weeks beforehand for design, planning, structural calculations and party wall notices. From first conversation to finished room is realistically nine to twelve months, most of which is consent rather than construction.