What does an extension cost?

There is no honest answer in a single figure to the question of what an extension costs. Anyone who gives you one is pricing a house he has never seen from the inside. What you can do is understand what that price is made of. An extension is not something with a price tag on it, it is a stack of cost items that falls differently at every house. Once you know that stack, you can see for yourself why one quote comes out higher than another, and you steer in the choices instead of in the quality. How I build an extension or annexe is on the service page itself.

The items an extension is made of

Groundwork and foundations

Everything starts below ground, and that is straight away the item with the most uncertainty. How much the ground can carry, how deep the foundations have to go and how the new foundation meets the existing one are three questions you only have the full answer to once the ground is open. On top of that, the soil you dig out has to go somewhere, through a passage, round the back or over the roof. If the garden is hard to reach, the difference is not in the material but in the hours.

This is the layer you will see nothing of later, and the one that weighs heaviest in the figure at the bottom of the quote.

The shell

Above that comes the body of the building: the walls, the insulation and the structure. Brickwork or timber frame is the first choice here, and those two run differently on cost. What matters more is what happens to the existing facade. If a piece of load-bearing wall comes out, a steel beam goes in to take over what that wall was carrying. That means a structural engineer doing the sums, steel ordered to size, and a moment when that steel has to get inside.

That one choice, going through a load-bearing wall or not, shifts the budget more than a couple of square metres of extra floor space.

The roof

A flat roof is simpler and cheaper than a roof tied into the existing one. Either way there is more to it than the covering alone: the structure, the insulation, the edge detailing and the rainwater drainage. People forget that last one every time, while the water that lands on your garden now ends up on a roof and has to go somewhere. If a skylight goes in, you are not only buying the glazing but the structure around it and the detailing that keeps it watertight. That carries on into roofing and roof renovation on the existing house.

The facade finish

The outside can be fair-faced brickwork, render or cladding, partly a matter of taste and partly a matter of maintenance. What you choose here decides how the extension sits against the rest of the house: matching the existing brick is one thing, deliberately breaking with it using facade cladding is another. Both can work well. What rarely works well is a join that is half the same and half different.

Frames and glazing

This is the item you steer the look with most, and where the price moves fastest: a facade with a wide set of sliding doors is a very different thing from a facade with a door and two windows. More glass means a heavier lintel, more weight on the foundation and dearer glazing. If you want light from above, you end up at a sunroom or a skylight, with a budget of its own.

The services

Electrics, water, waste, heating and ventilation. If the extension stays an extension of the living room with a few sockets, this is a modest item. If the kitchen moves in with it, the character of the work changes: the waste has to run to a fall, that fall sets the floor build-up, and the pipework has to come through the existing house to its new position. Choose underfloor heating and the floor build-up and the heat source hang off that.

The finish inside

Plaster, floor, skirtings, painting and the join between old and new. This item gets forgotten most often, because in your head the extension is done as soon as it is closed in. In practice the old part of the room almost always comes along: a floor running through from old to new asks that the existing floor is as level as the new one, and that falls under flooring and tiling.

Which choices matter most

Of all the decisions there are four that really move the budget. Whether the facade that opens up is load-bearing. Whether the roof is flat or tied in. How much glass goes in the new facade. And whether the kitchen moves with it. The rest is refinement.

What people get backwards: they shift things around in the visible layer because it is tangible, while the gain sits in the invisible choices. An extension that comes out a little less deep but keeps the same sliding doors saves you little. A flat roof instead of a tied-in roof saves you a lot, and in daily use you barely notice it.

Where people misjudge it

The price per square metre. A handy rule of thumb, a poor way to compare. The most expensive parts do not scale with the floor area: a foundation, an opening with a beam and a set of sliding doors cost roughly the same on a modest extension as on a generous one. Spread those fixed items over few metres and a small extension works out dear per metre.

The existing house joins in. The new space is rarely where the work stops. The old room gets darker, the floor has to run through, the layout no longer works. If that runs on, it becomes a renovation of the ground floor and not just an extension. Better to know that beforehand than halfway through.

The items outside the contract sum. The planning application, the structural calculations, the council fees, the kitchen units and putting the garden back after the groundwork. Not hidden costs, but costs all the same.

What to ask when you compare quotes

Put two quotes side by side and ask each of them the same questions. Is the opening in the existing facade in there, including the beam and the making good around it? What has been assumed about the foundation, and what happens if the ground turns out worse than expected? Which type of frame and which glazing have been priced, and to what brand or quality? Is the finish inside included, or does the quote stop at a shell? Who does the services, and does the work in the existing house come with it? Who takes away the soil and the building waste? And is there anything anywhere saying what is not included?

A quote that answers those questions itself is usually dearer than a quote that leaves them open, and almost always the more honest of the two. The difference between two figures is usually a difference in scope, not in price.

What I do before there is a figure on paper

I come and look, measure up and assess the structure and how reachable the garden is. During that visit we go through the four choices above, because those are the ones you can turn before anything is fixed. After that you get a quote item by item, which also says where the uncertainties sit. Tucking uncertainty away in a round total is easy, but then it comes back later as extra work.

From the groundwork to the last skirting board you have one point of contact with me, and how I approach an extension is on extensions and annexes.