Is a Loft Conversion Cheaper Than an Extension?
The rate gap is real, and what closes it is everything the rate leaves out.
Per square metre of new floor a loft conversion is usually cheaper, commonly £1,500 to £2,200 against £2,200 to £3,000 for a single storey extension, because the loft already has walls, a roof and foundations under both. The gap narrows once you add the staircase, the fire protection a new upper floor triggers, and the floor area the stair takes off the storey below. Compared per usable square metre, after the sloping ceilings at the eaves, the two can finish level. And if only one of the two gives you the room you actually need, the price comparison is beside the point.
Per square metre of new floor, usually yes. A rear dormer conversion commonly runs £1,500 to £2,200 per square metre of building work against £2,200 to £3,000 for a single storey extension, because the loft already has walls, a roof, and foundations under both that were sized for a house.
The gap is real. It is also smaller than it first looks, because the loft pays for a staircase, fire protection and floor area taken off the storey below, and none of that shows up in a rate. On some houses it closes completely.
| Item | Loft conversion | Single storey extension |
|---|---|---|
| Rate | £1,500 to £2,200 per square metre | £2,200 to £3,000 per square metre |
| Groundworks | None | Trench, slab and a structural opening |
| Staircase | Takes floor off the storey below | None |
| Fire work | A protected stair above 4.5 metres | None |
| Usable floor | A standing strip with wedges each side | Full head height across the footprint |
| Garden | Untouched | Spent |
Why the loft starts cheaper
Nothing goes in the ground, and the ground is where an extension spends its first several thousand pounds.
An extension buys its whole envelope from scratch: a trench, a slab, four walls, a roof, and a structural opening back into the house. Around Colchester most of the ground is shrinkable clay and the older stock has mature trees close to it, so foundation depth is the largest single variable in the price, and it is settled by what is under the lawn rather than by anything about the room. What a single storey extension costs sets that out in detail.
A loft has none of that. The roof is on, the walls are up, the drains are where they always were. What you are buying is a new floor, new structure to replace what the roof was doing, insulation, a stair and a finish.
The staircase is the cost the comparison forgets
It is joinery, a structural opening, and a decision about which room downstairs gets smaller.
A compliant flight needs a run, a landing at the top and headroom over both, and it has to arrive somewhere that leaves a usable plan. On most semis it comes off the existing landing and takes a slice out of the smallest bedroom. On a terrace with a narrow stairwell it has to turn, which needs winders and more floor again. The loft gives you a room upstairs and quietly takes area off the storey below, which no per square metre figure will show you. Whether it costs you a bedroom outright is a plan question, and it is answered in will I lose a bedroom to the loft stairs.
Fire work has no equivalent at ground floor
Adding a floor at the top changes what the staircase has to do all the way down.
Once the floor of the new room sits more than 4.5 metres above ground level, the stair becomes a protected escape route: fire doors onto it, upgraded linings, and a route that delivers you to a final exit rather than to a window. The trigger is that height, not a count of storeys, which is why a bungalow conversion and a conversion above two existing floors can land on opposite sides of the same rule.
An extension at ground floor triggers none of it. This is the clearest item that exists on one side of the comparison only, and it is the one most often missing from the cheaper of two quotations.
The rate only applies if the head height does
Below a certain height the roof has to change, and the loft stops being the cheap option.
As a working guide you want around 2.4 metres of existing height from the top of the ceiling joists to the underside of the ridge. Roughly 300mm goes at the top for insulation and lining and about 150mm at the bottom for the new floor. Below about 2.2 metres the conversion still works, but only by raising the ridge or building a dormer to create height where it is needed, and both are a different order of cost.
What the roof is made of matters just as much. A traditional cut roof has an open void you can walk into. A trussed rafter roof, which is most estate housing from the 1970s onwards, is a web of thin timbers that all have a job, and every one of them has to be replaced by new structure before any of it comes out. Cut roof or trussed rafters explains how to tell which you have in about five minutes.
Compare usable metres, not footprints
A loft floor and an extension floor are not the same square metre.
An extension gives you full head height across the whole footprint. A loft gives you a strip down the middle you can stand in and a wedge at each side where the ceiling comes down to meet the floor. A conversion with 30 square metres of floor may only offer 20 you can properly use, so a rate that looked 30 per cent cheaper is closer to level once you divide by the space you will actually occupy.
The wedges are not wasted, and a good plan puts the bed and the storage into them deliberately. But they are why two projects quoted at the same figure produce very different rooms.
When the extension wins outright
Three situations reverse the answer before any rate is applied.
A bungalow with a shallow trussed roof and nowhere obvious for a stair often costs more per usable metre upstairs than out the back. A house where the only compliant stair position eats a bedroom loses the room it just gained. And a family that needs a bigger kitchen is not in this comparison at all, because a loft cannot deliver one.
There is also the garden. An extension spends it and a loft does not, which on a small modern plot is a genuine cost even though nobody prices it.
When the comparison is the wrong question
You are not buying square metres. You are buying one specific room.
The rate comparison only means anything if both routes would give you the room you actually need. A fourth bedroom with an en suite is a loft question. A kitchen you can eat in is a ground floor question. Asking which is cheaper per square metre when only one of them solves the problem is how people end up with a converted loft they use for storage.
Working out which route your house and your problem point at is the job of extending out, up or into the roof. Price the two only once that is settled.
What to check before either gets priced
Four things, all measurable in an afternoon, decide most of the difference.
- Head height at the ridge, measured from the ceiling joists rather than from the boards.
- Whether the roof is cut or trussed.
- Where a stair could land, and what it takes from the room it lands in.
- The planning history for the address, because a previous extension may already have spent part of what either route relies on.
We survey all four before quoting, and we will say when the answer is that one of the two routes does not work on your house. That is a more useful thing to be told early than a rate.
These figures are building work only. Fees, statutory charges and VAT sit outside them, and both routes carry them.
Last reviewed August 2026.
Thinking about the job itself?
Most people who call us already have drawings and permission and want a builder to price the work and build it. Either way the survey and the written quotation cost nothing.