How Much Does a Single Storey Extension Cost?
A rate, a worked figure, and an honest account of why the top of the range sits forty per cent above the bottom.
A single storey extension in the Colchester area typically runs £2,200 to £3,000 per square metre for the shell and a standard finish, which puts a 20 square metre rear extension at roughly £44,000 to £60,000 of building work. VAT, drawings, structural calculations, planning and Building Control fees, party wall surveyor fees and the kitchen or flooring sit outside that. Most of the distance between the bottom and the top of the range is foundation depth on shrinkable clay and the specification of the glazing, not the quality of the finish. Small extensions cost more per square metre than large ones, because setting up, the drain connection, the scaffold and the structural opening do not shrink with the floor area.
Around £2,200 to £3,000 per square metre for the shell and a standard finish, which puts a typical 20 square metre rear extension at roughly £44,000 to £60,000 of building work. VAT, professional fees and the statutory charges sit outside that figure, and they are not trivial.
The rate is the easy half of the answer. The useful half is why the top of that range sits close to forty per cent above the bottom, because the distance between the two has very little to do with how good the finish is.
The gap is ground and glass
Two items decide where in the range a single storey extension lands, and both are settled before a brick goes above ground.
Foundation depth is the largest single variable around Colchester, because most of the ground here is shrinkable clay and mature trees are common on the older stock. On a twenty to thirty square metre footprint with a clear site and reasonable access, groundworks from setting out through to a finished insulated slab commonly run £9,000 to £16,000 at a depth of about a metre. The same footprint at two and a half metres, or on a piled solution beside a protected tree, can run £25,000 to £45,000 and sometimes beyond. Nothing about the room changes. The number moves by the price of a small car because of what is under the lawn.
That is also why single storey work carries a higher rate per square metre than a two storey extension does. The entire cost of getting out of the ground is carried by one floor. What the clay here actually does, and how the depth gets decided, are both worth reading before you accept anybody’s assumption about it.
Glazing is the other end of the gap. A pair of standard patio doors and a full width run of high specification sliders with slim frames and solar control glass are the same hole in the wall and a five figure difference in what fills it. It is the item most often left vague in a cheap quotation, written as “doors to rear” with no width, no frame material and no glass specification stated. Ask for all three in writing before you compare anything.
Why a small extension costs more per square metre
The rate is an average across a whole job, and a large part of that job does not scale with floor area at all.
Setting up and protecting the site, the drain connection, the scaffold, the structural opening into the house, the steel and its padstones, and making good where new work meets old are much the same on a 12 square metre extension as on a 25 square metre one. Spread across less floor, they read as a higher rate.
A small extension is therefore rarely the bargain it looks on a spreadsheet, and the marginal square metre is usually the cheapest one you will ever buy. Worth knowing before you shave a metre off the depth to save money. What you are permitted to build is a separate question, answered in how deep a single storey extension can be.
The structural opening is priced on its own
Most single storey extensions take out some or all of the rear wall, and in an honest quotation that work sits outside the per square metre rate. A structural opening commonly runs £3,000 to £8,000 or more, depending on span, load and what is found under the bearings. A three metre opening carrying a modest load sits at the bottom of that. A full width removal with a deep beam brought in through the house and a new pad dug by hand under one bearing sits at the top and occasionally past it. The beam depth is set by span and load, so it is one of the few items on a quotation that is not open to preference.
Access, the money you cannot point at afterwards
A house with a side gate wide enough for a barrow is materially cheaper to build on than a terrace where every load of muck and every pack of blocks goes through the hallway on boards. The extension is identical. The labour is not, and neither is the programme, because a skip that cannot be reached from the garden turns into wheelbarrow trips through a house somebody is living in.
It is the one variable that produces nothing visible when the job is finished, which is why a cheap quotation quietly leaves it out. On a mid terrace it is the reason a side return is priced above a plain rear extension of the same area.
What the rate does not include
The exclusions are the same across every extension type here, and they are the reason a quotation and a budget are two different documents.
Budget separately for VAT, kitchen units and appliances, bathroom sanitaryware, flooring and decoration, the drawings and structural calculations, planning and Building Control fees, party wall surveyor fees where a neighbour dissents, a build over agreement where one is needed, and service diversions. The full list, alongside what does sit inside the rate, is on the extension costs page, and there is no second version of it.
Building control is a modest figure set by a published scheme. A party wall surveyor is not modest, and on a semi or a terrace the building owner normally pays for every surveyor on the job, including the one the neighbour appoints. Drawings, structural calculations and statutory fees together commonly come to a few thousand pounds, and they are payable whoever builds it.
Where a cheap quote gets its number
A figure well below the range is usually not a cheaper builder. It is a shallower assumption, and the assumption is nearly always the foundation. A price built on a one metre dig, on ground that turns out to need two and a half, will be the most attractive quotation you receive, and the difference reappears as a variation once the trench is open and there is nothing left to compare it against.
Before you set two numbers side by side, check that each one states the foundation depth allowed for, says whether structural calculations are in or out, names the Building Control route, and carries an exclusions list. A quotation with no exclusions section is an opening position rather than a price. What else to ask a builder goes further.
Turning the range into a figure
Four things convert this into a price: the floor area, what the ground is doing, whether materials can get round the side of the house, and the glazing specification. A survey settles all four in an afternoon, and the quotation that follows prices the job somebody has actually stood in rather than an average one.
Where an assumption has to be made, the quotation should say so and say what happens if it proves wrong. Foundation depth is nearly always the one. How the job runs from first drawing to completion certificate, and how long it takes, are set out separately.
These figures are indicative for the Colchester area at the review date below, and they move with materials and labour. Use them to sense check a quotation, not as one.
Last reviewed August 2026.
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