What actually drives the cost of an extension

Clients think extensions are priced by the square metre. Builders know better: two extensions of identical floor area can be tens of thousands of pounds apart, and both prices can be honest. Here are the drivers that genuinely move the number, roughly in order of how hard they move it.

Size matters, but not the way clients think

Floor area is the starting point, not the answer. Cost per square metre falls as extensions grow, because the expensive parts of an extension are its edges and its connections: foundations, walls, roof, the structural opening into the house, drainage and services. Doubling the floor area does not double any of those. A tiny extension carries all the fixed costs of a big one, which is why small jobs look shockingly expensive per square metre and why quoting from someone else's per-metre rate is a fast way to lose money on small work.

The eight drivers that move the price

Notice what is missing from the top of the list: floor area. It sets the scale, but the drivers set the price.

The ground: the driver you cannot see

Everything above the damp course is visible on the drawings and priceable at your rates. The ground is a negotiation with reality. Clay near trees, made-up ground, a Victorian sewer running exactly where the foundations want to go: each one turns a standard strip footing into an engineering exercise. Good practice is to price what the drawings and a trial hole justify, then fence the rest with clearly written assumptions and a named provisional sum. The full approach is in groundworks: pricing the part of the job you cannot see.

Openings, steel and glass: the glamour tax

The modern extension brief usually includes one wall of glass and one vanished wall between kitchen and extension. Both are achievable and both are expensive in ways that surprise clients. The vanished wall means structural steel, temporary propping, and careful making good; the wall of glass means a product that can cost as much per linear metre as the masonry it replaces, plus the lintel above it. When a client wants to understand why their neighbour's extension was cheaper, the answer is very often "smaller openings, standard windows". Pricing steel properly is its own discipline, covered in pricing structural steel on domestic jobs.

The inside decides the budget as much as the shell

A weathertight shell with plastered walls is the predictable half of the job. What goes into it is the volatile half. Suppose your shell for a modest single-storey extension works out at 55,000 GBP at your rates. Put a mid-range kitchen, an island, underfloor heating and a utility room inside it and the project can add another 25,000 GBP or more; leave it as a plain garden room and it adds a few thousand. Same shell, wildly different totals. Quote the shell and the fit-out as visibly separate sections so the client can see which half of the budget is construction and which half is choices.

Putting the drivers to work in your quoting

The practical payoff of thinking in drivers is speed and defensibility. When an enquiry arrives, walk the drivers in order: ground risk, opening size, roof form, glazing, fit-out, spec, access, region. Ten minutes of driver questions tells you more about the eventual price than an hour of measuring, and it tells the client that you price with reasons rather than vibes.

It also maps directly onto how modern tools work. QS Quoter takes a job description or the architect's drawings and produces a fully itemised quote at your day rates, your materials uplift, your regional settings and your margin, with every driver showing up as visible lines you can edit. The client gets a clean, professional copy; you get a private cost breakdown showing where the money and risk sit. When the client swaps bifolds for French doors, you adjust the lines, not the whole evening.