Steel frame extension kits
Two different products are sold as steel frame extension kits. One is a set of light galvanised panels that go up like a timber frame. The other is a skeleton of heavier posts and beams, chosen when the back of the room is mostly glass. Know which one you are being quoted for.
In the kit
- Wall frames or posts and beams, cut and drilled
- Roof joists or purlins
- Bolts, screws and base fixings
- Bracing
- Structural calculations and a marking plan
- The declaration of performance for the steelwork
Not in the kit
- Insulation and sheathing boards
- Windows and doors
- Cladding and roof covering
- Internal lining
- Lifting gear for heavy beams
Light gauge panels
Light gauge steel is thin galvanised sheet rolled into C-shaped studs and screwed or riveted into wall frames. A panel weighs less than its timber equivalent, arrives dead straight and stays that way: steel does not shrink, twist or take up moisture. Frames are screwed together on site with a cordless driver. No welding is involved.
For an ordinary single-storey room, light gauge does the same job as timber frame. People choose it for straightness, because it will not burn, or because the supplier they like works in steel.
A hot-rolled skeleton
The second kind uses the same heavy sections as the beam over a knocked-through wall: posts at the corners, beams between them, and infill walls of timber or light steel. Its strength is the opening. A steel beam can span 5 m or more of sliding doors without a post in the middle, and slim posts leave more glass on a corner.
The beams are heavy. A 5 m beam cannot be walked down a side passage and lifted by hand. Plan for a hired lifting machine or a small crane, and check the route in before ordering.
Keeping a steel frame warm
Steel carries heat several hundred times faster than timber. If the insulation only sits between the studs, each stud becomes a cold stripe through the wall, and warm indoor air can condense on it. The fix is a warm frame: most or all of the insulation goes on the outside of the steel as a continuous layer, so the frame stays at room temperature.
Ask to see the wall build-up and its U-value calculation, which must account for the steel. New extension walls in England need 0.18 W/m²K. A kit that only offers insulation between the studs is not finished thinking.
The certificate to ask for
Structural steelwork sold in Great Britain must be UKCA or CE marked under BS EN 1090-1. The fabricator holds a factory production control certificate naming an execution class, and for a house extension that is normally Execution Class 2. Ask for a copy of the certificate and the declaration of performance for your frame. A reputable fabricator sends them without fuss, and building control may ask you for them.
How steel frame kits are priced
We do not print a per square metre figure for steel. The national price guides we rely on give one range for flat-pack extension kits and finished prices for timber frame and SIPs, but none for steel. The general kit range on our price page is a rough marker at best.
Steel kits are priced from the drawing, by the weight of steel and the number of connections. Two things move the price most: the width of the largest opening, which sets the size of the biggest beam, and whether the frame has to be hot-dip galvanised after fabrication. Send the same drawing to each supplier and ask them to state the steel tonnage, so you can see whether you are comparing like with like.
Living with it afterwards
Nothing can be nailed to steel. Linings, battens and brackets go on with self-drilling screws, and cables passing through studs need grommets so the sheath is not cut. Electricians and plasterers who mainly work on houses may not have done it before. It is not difficult, but mention the frame when you ask them to quote.
Where our information comes from
- GOV.UK: Approved Document A, structure
- GOV.UK: Approved Document B, fire safety
- GOV.UK: Approved Document L, conservation of fuel and power
- GOV.UK: Building regulations approval
- Steel Construction Institute
Rules quoted are for England unless we say otherwise. Wales follows closely; Scotland and Northern Ireland have their own building standards.
Steel frame kits: common questions
Is a steel frame extension kit cheaper than timber?
Not as a rule. For a plain room the two are close, and timber is often the lower quote. Steel earns its keep when the design has a long span or very slim supports that timber could not manage without bulky posts.
Will a steel frame rust?
Light gauge sections are galvanised before forming, and a frame kept dry inside a warm wall will outlast the house. The risks are at the base, where the frame must sit above the damp-proof course, and at any cut made on site, which should be touched up with zinc-rich paint.
Does steel solve the fire rules at a boundary?
Only partly. The frame itself does not burn, but a wall within 1 m of the boundary still has to resist fire as a whole, and bare steel loses strength in heat. The wall needs fire-rated boards on both faces whatever the frame is made of.
Can I put up a steel frame kit myself?
A light gauge kit, yes: it is screwed together much like flat-pack shelving at a larger scale. A hot-rolled skeleton is a job for people with lifting equipment and experience of bolting up steel.
Get a quote for your kit
A postcode, four short questions about the kit, then your phone and email. Most people finish in under a minute.
Extension Kits is a free introduction service, not a kit maker. We find a kit supplier that covers your area and send you its name first. Nothing is passed on until you say yes.
Thank you. Your request has reached us.
- We look for a kit supplier that delivers to your postcode and makes the system you chose.
- Within one working day we email or phone you with that supplier's name.
- If you are happy with them, we hand over your request and they price the kit. If not, nothing is passed on.