Single-Storey Extensions in Cambridge

A single-storey extension is the most common job we are asked to price in Cambridge, and the one where the difference between a good build and a cheap one is least visible once the plaster is on. This page covers what is actually involved, what drives the cost, and the decisions that are difficult to reverse later. For budgets and regulations, our house extension costs guide and Building Regulations guide go further than we do here.

Rear, side return or wraparound

Most single-storey extensions fall into one of three shapes, and the right one is usually decided by the house rather than by preference.

A rear extension runs the full width of the back of the house and is the simplest to build. It gives the most floor area for the money because the structure is straightforward and the roof is a single span.

A side return fills the narrow strip beside the original back addition on a Victorian or Edwardian terrace. It adds less floor area than people expect, but it transforms the light and the shape of a dark galley kitchen, which is usually the actual problem. It is more difficult to build: you are working in a confined space, often against a party wall, and access for materials is poor.

A wraparound does both at once. It is the most expensive per square metre because of the extra corner, the more complicated roof and the longer run of foundations, but it is far cheaper than doing the two jobs in separate years.

Opening up the back of the house

The moment that decides how the finished house feels is when the existing rear wall comes out. That wall is usually load-bearing, so it is replaced with a steel beam sized by a structural engineer and carried down through padstones onto piers or foundations that can take the load.

Two things are worth understanding before you choose how wide that opening is. First, a wider opening means a deeper beam, and a deeper beam eats into ceiling height unless it is set up into the floor above, which costs more. Second, the beam has to be got into the house and lifted into place, and on a terrace with no side access that is a genuine constraint rather than an excuse.

We prop before we cut, every time, and the props stay in until the beam is bearing properly. It is the least photogenic part of the job and the part where corners are most often cut.

Foundations, and why the quotes differ

Foundations are the single biggest reason two quotations for the same drawing come back thousands apart. Until the ground is open, nobody knows exactly what is down there, so an honest quotation prices a defined depth and says plainly what happens if the trench has to go deeper.

The things that push foundations down in and around Cambridge are drains running through the footprint, a neighbouring extension with a deeper footing you have to match, and trees. Clay soil shrinks as roots draw water out of it, so proximity to a mature tree can mean a much deeper trench or a different foundation type altogether. None of that is visible when the quotation is written.

On the Great Shelford job the trenches went to roughly 1.4 metres and the depth was recorded against a tape as they were dug, with the existing footings exposed alongside so the new work could be related to what was already there. That record is what protects both sides if a question comes up later.

Flat roof or pitched

Most single-storey extensions get a flat roof, and a modern flat roof is not the flat roof people remember failing in the 1970s. A warm-deck build-up with a single-ply or rubber membrane, laid to falls, with the insulation above the deck rather than between the joists, is a reliable roof.

The advantage of flat is rooflights. A pitched roof off the back of a house often ends up with a low ceiling at the far wall and no useful glazing overhead; a flat roof with two or three good rooflights puts daylight into the middle of the plan, which is exactly where a deep extension goes gloomy.

A pitched roof is the better answer when the extension is visible from the street, when the planning officer expects it, or when the ceiling is going to be vaulted. It costs more and it takes longer.

Glazing, and the bifold question

Bifold doors are asked for more often than they are the right answer. They stack to one side and give you a fully open wall, which is excellent for perhaps ten days a year in this country. The rest of the time you are looking at a lot of frame, because every leaf has a stile down both sides.

A large sliding door gives more glass and less frame for the same opening, and the panels are bigger so the view is cleaner. A fixed window with a single door beside it is cheaper again and often better where the garden is small. It is worth deciding this on how the room will be used rather than on the brochure.

The floor

Ground floors in extensions are usually either a concrete slab or beam and block. Beam and block spans between the foundation walls rather than bearing on the ground beneath, which suits made-up ground and keeps the floor clear of the soil. It goes down quickly and it is checked for level as it is laid, because everything above it inherits any error left in it.

If underfloor heating is going in, it has to be decided before the floor is built, not after. So does the finished floor level, which needs to line through with the existing house unless you want a step you will trip on for the next twenty years.

What it costs and how long it takes

We do not publish a rate per square metre on this page, because the honest answer depends on the ground, the access, the roof and the glazing, and a number without those attached is worse than no number. Our extension costs guide sets out current Cambridgeshire ranges from published sources, explains what moves them, and shows worked examples.

On programme, a straightforward single-storey rear extension is usually a matter of months rather than weeks once work starts, and the design, planning and Building Regulations stages beforehand often take longer than the build itself. We give a written programme with the quotation rather than a verbal estimate on site.

Planning and Building Regulations

Many single-storey rear extensions can be built under permitted development rather than a full planning application, but the limits are specific and they are different if you live in a conservation area or the permitted development rights have been removed. Our permitted development guide and planning permission guide set out the process and what evidence to keep.

Building Regulations apply either way. They are a separate approval from planning and they are the one that governs structure, insulation, drainage, ventilation and fire safety. See the Building Regulations guide for how that runs alongside the build.

Common questions

Can I stay in the house while it is built?

Usually yes for a rear extension, up to the point where the existing wall comes out and the kitchen is disconnected. That period is the disruptive one and it is worth planning for rather than discovering.

Do I need a structural engineer?

If a load-bearing wall is coming out or a beam is going in, yes. The engineer sizes the steel and specifies the bearings; we build to their calculations and the Building Control inspector checks against them.

What about my neighbour?

If you are building up to or across the boundary, or excavating near their foundations, the Party Wall etc. Act 1996 is likely to apply and notice has to be served before work starts. It is worth raising early, because it takes time and it goes better as a conversation than as a letter.

Can you match the existing brickwork?

Usually closely, rarely perfectly. New brick against a weathered wall always reads cleaner at first. The gault brick matching case study shows how we course new work through to an existing Victorian terrace and what the result actually looks like.

Extensions we have built

The glazed rear extension in Highfields Caldecote and the Great Shelford job are the closest single-storey records on the site. For the wider service see house extensions, and for going up as well as out see two-storey extensions.

To put a first budget together, use our extension cost calculator. It returns a line-by-line range rather than a single figure, and it says plainly which costs need a written quotation.

Tell us what you would like to build and share any drawings or photographs you already have. We aim to respond to every enquiry within one working day with the most useful next step.