Steel Frame Extension
Wide openings, slim posts and a roof that spans the whole room. See what one costs, then get a price for your house.
Get a quoteSteel is what lets the back of a house open up without a row of brick piers in the way.
The frame carries the roof and the wall above on a few slim posts, so five or six metres of glass can stand where brickwork used to. It is made in a workshop to an engineer’s drawings and bolted up on your foundations in a day or two.
What a steel frame changes
Blockwork and timber can build most extensions. Steel earns its place when the design asks for one of these.
- An opening wider than about four metres, or doors that meet at a corner with no post.
- A thin roof edge and slender columns, where deeper timber would spoil the line.
- A wall or floor above that has to stay up while the room below opens out.
- A lot of glass. Steel is stiff enough to keep big panes and sliding doors running true.
- 4 mHow far a single-storey rear extension can reach from a detached house in England with no planning application. Attached houses get 3 m.
- 30 minThe fire resistance steel needs where it holds up a floor in a two-storey house. Board or intumescent paint provides it.
- 25%Glazing allowed, as a share of the new floor area, before building control wants an energy calculation.
From the permitted development guidance and Approved Documents B and L for England. Wales follows similar rules. In Scotland the limits differ and a building warrant comes first.
What a steel frame extension costs
Finished single-storey rooms, including VAT. A hot-rolled frame runs £2,200 to £3,000 per m² of floor. National figures and a guide, not a quote: London and the South East usually cost 20 to 40% more.
| Floor area | Hot-rolled steel frame | Light gauge steel panels | Glass box |
|---|---|---|---|
| 15 m² | £33,000 to £45,000 | £31,500 to £42,000 | £41,500 to £67,500 |
| 24 m² | £53,000 to £72,000 | £50,500 to £67,000 | £66,000 to £108,000 |
| 36 m² | £79,000 to £108,000 | £75,500 to £101,000 | £99,000 to £162,000 |
Engineer’s fees and building control are extra. VAT at 20% is included.
How the job runs
Drawings and calculations
A designer draws the room. A structural engineer sizes each beam, column and connection, and building control receives the numbers.
Fabrication
The frame is cut, drilled and welded in a workshop while the foundations go in at home.
Frame up
Steel arrives on one lorry and is bolted together in a day or two, by hand or with a small crane.
Walls, roof and glass
Insulated walls, the roof deck and glazing close it in. Inside, it finishes like any other build.
Before you ask for prices
Is steel dearer than brick and block?
Usually, by a few hundred pounds a square metre: masonry extensions run £1,800 to £3,000 per m². The gap narrows when the design has wide openings, because a masonry build then needs heavy steel as well.
Do I need planning permission?
The frame makes no difference; size, position and looks do. Many rear extensions fall under permitted development, but a metal-clad or mostly glass room may not match the house closely enough, and then a householder application costs £548 in England.
Will the steel make the room cold?
Not if the frame is kept inside the insulation. The places to watch are where a beam or post passes through the wall line; these are wrapped or fitted with thermal breaks.
Can the steel be left on show?
Yes, and exposed beams and columns are a look in their own right. They cost more, because the welds are dressed smooth and the fire protection has to be paint in place of plasterboard.
Get a quote for your extension
Three short steps. Your postcode, four questions about the job, then how to reach you.
We are an introduction service, not a steel frame extension specialist. We look for a single firm working in your area, give you its name, and hold your details back until you give the go-ahead.
Received, thank you.
- Within a working day we come back with the name of one firm that takes on steel-framed work in your area.
- Nothing goes to that firm unless you tell us to go ahead.
- If you do, they come out, measure and send their price. We send no marketing of any kind.