How Much Does It Cost to Extend a Bungalow?

Building out at ground level prices like any single storey extension, about £2,200 to £3,000 per square metre, so a 25 square metre addition is roughly £55,000 to £75,000. Going up into the existing roof is cheaper per square metre than that; taking the roof off to add a full storey is dearer than both, and it is the one route where a rate per square metre stops being a useful way to think about it.

Three routes, three different economics. What links them is that on a bungalow the roof is the dominant item in all three, which is not true of any other house type.

Why the roof drives the number here

A bungalow has more roof per square metre of floor than anything else you could extend.

A two storey house spreads one roof across two floors. A bungalow spreads one roof across one. Extend outward and you are buying a square metre of new roof for every square metre of new floor, at full price, with all its structure, insulation, covering, edges and rainwater goods. That single fact sits underneath most of the differences between this and the equivalent job on a semi.

The low eaves is the second consequence. There is no first floor wall to run a new pitched roof down from, so a rear extension on a bungalow either continues the existing roof plane outward, which is elegant and structurally involved, or sits as a flat or shallow roof tucked under the eaves, which is simpler and puts the daylight strategy on rooflights rather than on tall glazing. That is a design decision with a real price attached rather than a matter of taste, and it is settled on the drawings.

Route one: build out

The most predictable of the three, and priced exactly like a single storey extension because that is what it is.

The drivers are the usual ones and they are set out in what a single storey extension costs: foundation depth on shrinkable clay, access, structural span, glazing and roof type. Two of them tend to fall in your favour on a bungalow. Plots are usually wider with genuine side access, so materials get to the back without going through the house, and the structural spans are modest because there is nothing above the opening except a roof.

Two work against it. The roof area is high, as above. And the ground floor is the whole house, so there is nowhere to retreat to while it happens, which is a household cost rather than a building one but a real one.

Check the drainage before designing. A wide bungalow often has long, shallow drain runs across the back of the plot, with the inspection chamber sitting precisely where the new room wants to go. Where the run is shared, a build over agreement and a camera survey come with it.

Route two: into the roof

The cheapest floor area available, when the roof allows it, because the foundations and the walls are already paid for.

A dormer conversion commonly runs £1,500 to £2,200 per square metre, well below the ground floor extension rate, and the reason is simple: no groundworks. What moves it within that range is set out in what a loft conversion costs.

Two bungalow-specific checks decide whether that cheap rate is available to you. The first is head height under the existing ridge, which on a shallow pitched post-war bungalow is frequently not there. The second is what the roof is made of. A traditional cut roof has purlins and rafters that can be worked with. A trussed rafter roof, standard on later stock, is a web of timbers that cannot simply be cut out, and replacing that structure with beams and new floor joists is a substantial piece of work that a cut roof does not carry. It does not make the job impossible. It moves it toward the price of the next route.

The staircase also has to come out of the ground floor you already have, usually from a hall or a bedroom, so you lose floor area you have already bought.

Route three: take the roof off

This is where cost per square metre misleads badly, because a large part of the spend buys no new floor at all.

Removing the roof, disposing of it, weather protecting an open house, building up the walls, and putting a complete new roof back on are all substantial items, and none of them appears as extra floor area to divide into. Add the checks that come with the extra load: the existing walls and foundations were built for a single storey building, so what is under them is established by trial hole and calculation rather than assumed.

You should also assume you are moving out. A house with its roof off is not habitable, and the rehousing period is a genuine line in the budget rather than an inconvenience. For that reason, an honest comparison prices this route as a whole project, and treats any per square metre figure you are quoted for it with suspicion.

The fire regulation point that saves money

Adding a floor above a bungalow changes what the escape arrangements have to do, and the trigger is a height, not a storey count.

The Approved Document B trigger for the more demanding escape provisions is the height of the floor above ground level, with 4.5 metres as the line. It is not the number of storeys. A bungalow with a new room in the roof becomes a two storey house, but its upper floor often sits under that line, which can keep an escape window as the answer instead of a protected stairway running the full length of the plan.

That is a meaningful difference in fit-out cost, in doors, in linings and in how the ground floor plan has to work, and it is settled by measuring rather than by argument. How escape is assessed covers the detail.

What none of the three rates include

VAT, kitchen units and appliances, bathroom sanitaryware, flooring and decoration, drawings and structural calculations, planning and Building Control fees, party wall surveyor fees where a neighbour dissents, service diversions, and anything genuinely unforeseeable once the ground is open. The full list is on the costs page, and it does not change between routes.

Which route the plot usually decides

Cost per square metre rarely settles this. A wide plot with a deep garden and a shallow roof points outward. A steeper roof with real height under the ridge, on a plot with nothing left to build on, points upward into it. Taking the roof off makes sense in the narrow case: a house you want to keep, a plot you have run out of, a trussed roof and no head height.

The planning position often decides it first, particularly where a bungalow sits on an estate whose original permission carried conditions, or in a street where the roofline is what the character assessment is protecting. The full guide to extending a bungalow works through how that choice gets made.

These figures are indicative for the Colchester area at the review date below and move with materials and labour. Use them to sense check a quotation rather than as one, and get the roof structure and the ground looked at before anybody prices anything.

Last reviewed August 2026.