Light gauge steel frame extension

What light gauge steel is

Galvanised steel strip between roughly 1 and 3 mm thick is rolled, cold, into C-shaped and U-shaped sections. Studs are typically 100 or 150 mm deep and stand at 400 or 600 mm centres, exactly where timber studs would. A factory screws or rivets them into panels from a 3D model, with every opening and service hole already in place.

Hot-rolled steel is a different animal: I-shaped beams and columns with walls 6 mm thick and more, each carrying a large load on its own. A light gauge wall spreads the load across dozens of small members instead.

How an extension goes together

The slab or footings come first and have to be flatter and squarer than a bricklayer would need, because factory panels do not forgive a base that is 20 mm out.

Panels arrive numbered on a flatbed and are bolted to the base and to each other. Two or three people can carry most of them, so a crane is not always needed. A single-storey shell is often standing within a couple of days.

Insulation then goes on the outside of the steel, the arrangement engineers call a warm frame. This matters: steel conducts heat readily, and keeping the studs on the warm side of the insulation stops them forming cold stripes in the wall where condensation could collect. Over the insulation go a breather membrane and whatever finish you want, from brick slips and render to timber or metal cladding, or a full brick outer leaf.

Inside, plasterboard does two jobs. It is the wall finish, and it is the fire protection for the steel behind it.

Where it suits, and where it does not

Light gauge steel is a good answer in a handful of situations.

• Building over a garage or on top of an existing single storey, where low weight spares the walls and foundations below.
• Terraced houses with no side access, because panels can come through the house.
• Programmes where a fast, dry shell matters, for instance when the kitchen is out of action.
• Designs that need walls dead straight and stable for large tiles, fitted joinery or long runs of glazing. Steel does not shrink, twist or creep as it dries, because it never was wet.

It is less convincing for wide openings. A five-metre set of sliding doors needs a hot-rolled beam or goalpost whatever the walls are made of, so most light gauge extensions are hybrids: heavy steel around the big opening, panels everywhere else.

Two practical drawbacks are worth knowing. Changes on site are awkward, since moving a window means remaking a panel. And fewer domestic builders have used the system than have used timber, so the firm that supplies the frame often puts it up as well.

Cost against the alternatives

Our guide range for a finished light gauge steel extension is £2,100 to £2,800 per m² of floor, including VAT. A hot-rolled frame with infill walls is £2,200 to £3,000 per m², and brick and block £1,800 to £3,000 per m².

So panels are not the cheap option on materials. The saving is in time: fewer weeks of site labour, scaffolding and disruption. Try your own size in the cost calculator.

Rules and paperwork

Planning treats the extension the same whatever is inside the walls, so the usual permitted development limits apply in England and Wales. Scottish rules use their own classes, and a building warrant has to be in hand first.

For building regulations, the frame supplier’s engineer provides the structural calculations. Building control will also look at the wall build-up for insulation and condensation risk, and at the plasterboard specification for fire.

Ask one question early: does the system hold independent certification? A British Board of Agrément certificate or an assessment by the Steel Construction Institute makes life simpler with building control, and later with a buyer’s surveyor.

Light gauge steel, hot-rolled steel and timber frame compared