How Much Does a Two Storey Extension Cost?

Around £1,900 to £2,600 per square metre of total floor area for the shell and a standard finish. A 30 square metre footprint gives 60 square metres across two levels, which puts a typical project somewhere between £114,000 and £156,000 before VAT, fees and the exclusions below.

That rate is lower than the single storey rate and the total is much higher, which is the whole shape of this question. The interesting part is why the rate falls, and what stops it falling further.

Why the rate per square metre drops

You buy a second floor of area from one set of foundations, one roof and one scaffold.

Getting out of the ground is the most expensive part of any extension on Essex clay, and on a two storey extension that cost is divided across twice the floor. The same is true of the roof, the drain connection, the site set-up, the scaffold and the welfare arrangements. None of those double when a storey is added.

That produces the argument worth having early. If you are already digging foundations for a single storey rear extension, the marginal cost of the upper floor is far less than the cost of the ground floor was, because the expensive items are already bought. Whether it makes sense is a question about the house and the planning position rather than the arithmetic, but the arithmetic is not the obstacle people expect.

What stops it falling further

Three things claw money back, and all three are structural or high up.

The structure gets heavier. A ground floor opening that carries a floor is one thing. A ground floor opening carrying a floor, a wall and a roof above it is a deeper beam, heavier padstones, and sometimes load taken down to its own new foundation. The beam depth is set by span and load, and no amount of preference changes it.

The foundation may go deeper anyway. Bearing pressure under a two storey wall is higher than under a single storey one, so the engineer’s design responds to the extra load as well as to the clay and the trees. The depth is not a copy of the neighbour’s job. It comes from a trial hole and a calculation.

The roof junction is real work. Tying a new roof into an existing one is where the design effort and a good deal of the labour goes, and it is the detail that decides whether the extension reads as part of the house or as something bolted on. A hip taken back, a valley formed, new lead work, and the cavity trays where the new roof abuts the old wall. None of it is visible from the garden when it is right, and all of it is expensive to put back when it is wrong.

The upper floor is not empty space

A first floor is bedrooms and usually a bathroom, and a bathroom is not priced like a bedroom.

Sanitaryware, a soil connection, a waterproofed floor, extract ventilation and a heated towel rail put an en suite well above the cost per square metre of the room next door. Sanitaryware itself sits outside the building quotation, along with the kitchen units downstairs.

Then there is the existing house. A two storey extension usually means altering the landing, sometimes moving a staircase, and always making good across two floors of internal wall where the old external wall used to be. Rooms you were not extending get plastered, rewired at the edges and redecorated, and that work is inside the programme whether or not it was inside the budget.

The costs that arrive with the second storey

Two storeys change the statutory position more than they change the building.

Two storey permitted development rights exist, but the depth limit, the distance required from the rear boundary and the roof pitch condition are narrow enough that many schemes fall outside at least one of them, which means a householder planning application and its fee. Assume the application until the position on your address has been checked, not the other way round. What the application costs is answered separately.

Party wall work is close to certain on a semi or a terrace, because the excavation and often the beam bearings engage the shared wall at both levels. The building owner normally pays for every surveyor involved, including the neighbour’s, so that figure belongs in the budget from the start rather than in a surprise letter.

Scaffold is up for longer and covers the whole elevation. Building control charges are banded by floor area, so the new work sits in a higher band than a single storey extension of the same footprint would. On the full plans route it also arrives as two invoices, a plan charge and an inspection charge, which is worth knowing before the second one lands.

What the range excludes

VAT, kitchen units and appliances, bathroom and en suite sanitaryware, flooring and decoration, professional fees for drawings and structural calculations, planning and Building Control fees, party wall surveyor fees, service diversions, and anything genuinely unforeseeable once the ground is open. The list, with what is inside the rate, is on the costs page.

One item people forget entirely: adding this much heated floor area puts a substantially larger load on the existing boiler. Checking that while the walls are open is cheap. Discovering it in the first cold snap afterwards is not.

Against the alternatives

If what you actually need is one more bedroom, a loft conversion is usually the cheaper route per bedroom, takes no garden, and is less disruptive to the ground floor. On a hipped interwar semi a hip to gable conversion produces a genuine double with an en suite for considerably less. The figures for that are on their own page.

A two storey extension earns its number where you need space on both floors, where the roof genuinely will not work because of head height or structure, or where the ground floor has to grow anyway and the upper storey is comparatively cheap once the foundations are going in. It is also the honest answer where the ground floor layout is the real problem, because a loft conversion does nothing about a small dark kitchen.

Turning the range into a figure

Total floor area across both levels, the ground conditions, the spans at each level, the roof form and the access. With those five, a quotation prices the actual job rather than an average of jobs. Where an assumption is unavoidable, it should be written down along with what happens if it is wrong.

How a two storey extension is built and managed, including the programme end to end, is set out in full elsewhere. These figures are indicative for the Colchester area at the review date below and move with materials and labour, so use them to test a quotation rather than to replace one.

Last reviewed August 2026.