What Drives Single Storey Extension Cost

What is under the ground

Foundations are the least predictable element of any extension, and the one no honest builder can fix before knowing the site. Clay subsoil with mature trees nearby means deeper foundations. A drain crossing the footprint means either a diversion or a bridging detail agreed with the water authority.

Sloping ground means retaining and more spoil to remove. None of these are optional, and all of them are decided by the site rather than the design.

Soil type and depth to bearing stratum · Trees on or near the boundary and their species · Existing drainage runs and manholes within the footprint · Access for muck away and materials, which drives labour hours · Level changes across the plot

Opening up the back of the house

The moment that makes an extension feel like one is the removal of the original rear wall. That is a structural operation: steel beams sized by an engineer, padstones, temporary propping and often a goalpost frame where a corner is opened.

The wider the opening and the more of the wall that goes, the heavier the steel and the more involved the installation. A full width opening with no down stand is a very different price to a wide doorway, and it is worth being clear which you are pricing.

Glazing, the visible variable

Glazing is where budgets are made and lost. A pair of french doors and a window is one thing. A run of bifolds or a large sliding door with slim sightlines and a structural rooflight above is quite another, and the difference can be a substantial share of the whole project.

It is also the decision that most changes how the room feels, so it deserves deliberate attention rather than being trimmed at the end when the money is short.

What is inside it

An empty shell is a fraction of the finished project. Most single storey rear extensions are kitchens, and the kitchen, appliances, worktops, flooring, underfloor heating, lighting and decoration are often close to the cost of the building work itself.

Treat the fit out as its own budget line from the start. Extensions that run over almost always do so on the inside rather than the outside.

The efficiency of size

Cost per square metre falls as the extension grows, up to a point. The expensive parts, the structural opening, the foundations reaching the bearing stratum, the drainage connection, the site setup, are largely fixed regardless of whether you build three metres or four.

That is why an extra metre of depth frequently costs far less proportionally than the first three, and why it is worth pricing two sizes rather than assuming the smaller one is the sensible saving.

Frequently asked questions

Why can nobody quote before a survey?

Because foundations and drainage are decided by the site, not the drawing. A price given without seeing the ground is a guess that will be revised once the ground is open.

Are bifolds worth it over french doors?

They open the room to the garden in a way french doors do not, but they are a significant line in the budget. If money is tight elsewhere, a large sliding door or a well proportioned set of french doors with a rooflight often gives more light for less.

Does the kitchen come out of the same budget?

It should be budgeted separately and explicitly. Building work and fit out are different scopes, and mixing them is the most common reason a project feels like it has overrun.

How much does the extension size change the price per square metre?

Meaningfully. The fixed elements are spread over more floor as it grows, so the marginal metre is usually cheaper than the average metre.