Single vs double storey extensions: where the money differs

Here is the piece of estimating logic every homeowner finds surprising and every good builder knows in their bones: a double storey extension is nowhere near double the price of a single storey one. The reason is simple once you see it. Some of the most expensive parts of an extension are built once, however many floors sit on them.

The costs you only pay once

Think about what the second storey actually reuses. The foundations are already dug, poured and inspected. The roof still gets built once, just one floor higher. The drainage connection happens once. The scaffold is already standing. Site set-up, welfare, skips, the crane day for the steels, the design fees, the building control application: one each. These are the heavyweight, disruption-heavy items of the whole project, and the second storey inherits them almost for free.

This is why estimators talk about the second storey as the cheap floor. The dig and the lid, the two ends of the building, are the costly parts. The middle is mostly walls, joists, windows and finishings, which are comparatively straightforward money.

The costs that genuinely double

The second storey is not free, of course. It adds a full lift of external walls with their insulation and openings, a structural first floor, and another set of rooms to wire, heat, plaster, floor and decorate. If the new upstairs rooms include a bathroom, the services block grows the way it always does. And the programme stretches by several weeks, which quietly adds prelims: every extra week is scaffold hire, supervision and site costs, a mechanism we unpack in how programme length changes your price.

There are also a few costs that only exist because of the second storey. The foundations may need to be designed heavier from the start. Party wall matters get more involved when you are building at height near a neighbour, which is worth reading up on in party wall basics. Lifting materials to first floor level takes either plant or patience. None of these are dramatic on their own; together they are the honest price of the extra floor.

A worked comparison, with made-up numbers

To make the shape concrete, suppose a single storey rear extension prices at £70,000 through a proper bill of quantities. Now suppose the client asks: what if we go up as well? The shared items, perhaps a third or more of that bill, barely move. The wall, floor, window and finishes lines are added for the new storey, the services grow, and a few weeks of prelims arrive. Depending on specification, the double storey version might land around half as much again rather than twice the number. Those figures are an illustration of the mechanism, not market rates: the exact ratio on a real job depends on the ground, the spec upstairs, and everything else covered in our guide to what an extension costs in 2026.

The costs that are easy to miss on the way up

A few second storey items deserve their own attention because they hide between the obvious lines. Bedrooms upstairs usually mean tying new rooms into the existing landing, which can involve altering the existing roof where the two buildings meet, a junction that is fiddly, weather-critical and priced too casually on many quotes. Windows at first floor level need safe access for fitting and, later, for cleaning, which can shape the glazing choice. The extra load may change the foundation design even though the foundations are dug only once, so the engineer's input belongs at quote stage, not after acceptance. And the disruption profile changes: a single storey rear extension can often be built with the family living almost normally, while a double storey job puts scaffold, noise and trades at bedroom windows for weeks longer. None of these change the argument. They are simply the honest lines that stop the double storey quote being a single storey quote with a multiplier.

What this means for the client conversation

For builders, this asymmetry is a genuinely useful thing to explain at the kitchen table, because it reframes the decision. The client who wants a single storey now and imagines adding a second one later is choosing the most expensive possible route: two sets of scaffold, two programmes, a roof built and then dismantled. If there is any realistic chance they want the space, pricing both options side by side is a service, and it often converts a bigger job.

The way to do that credibly is with two measured quotes, not one number and a shrug. Show the shared costs sitting in both versions, show what the second storey genuinely adds, and let the client see the mechanism themselves. An itemised bill turns "trust me, it is better value" into something they can check.

Pricing both options without doubling your evenings

Pricing two versions of the same extension repeats much of the same take-off and document work. QS Quoter assists that comparison: describe the job or upload the drawings once, then review each option as an editable bill of quantities at your own rates. You get a client copy for each option and a private copy showing your costs and margin, with every line editable before anything is sent.