Most home extensions use steel, and it usually comes in one of two forms. The first is beams, often called RSJs, that carry the load where you open up a wall or span wide doors. The second is a full steel frame, used when an opening or layout is too large for masonry to span on its own. Which one you need is decided by a structural engineer, not by the builder or by guesswork. Here is where steel fits into an extension, the difference between supply-only and supply-and-fit, and the process from calculation to final bolt.
Where steel comes into an extension
In a typical extension, steel does the structural work that brick and block cannot do alone:
- Carrying loads over openings. When you remove part of an existing external wall to connect the extension to the house, the wall above (and often a floor or roof) has to be supported. That is what a beam does. The same applies over wide patio doors or bi-folds.
- Spanning large openings. Modern extensions favour big glazed walls and open-plan layouts. Once an opening gets beyond what masonry can span, you need steel, and beyond what a single beam can carry, you need a frame.
- Wrap-around and corner extensions. Removing two walls or a corner of the house leaves nothing for masonry to sit on. A steel frame (beams and columns working together, sometimes a goalpost) holds the structure above so the corner can be opened up.
So a small single-storey extension might need just one or two beams, while a large wrap-around or a glass-heavy design needs a frame. For the frame side, see our steel frame buildings service.
Beams (RSJs): the workhorse of most extensions
The most common piece of steel in an extension is a beam over an opening. When you knock through to join the extension to the kitchen, the masonry above needs support, and a correctly sized beam provides it. A run of bi-fold or sliding doors does the same job in the new wall.
The size of that beam is not a standard part you pick off a shelf. A structural engineer calculates the load above the opening and specifies the section (its depth and weight per metre) and length. For more on how beams over openings are priced and fitted, see our guide to RSJ installation cost, and for the supply-and-fit service itself, RSJ supply and fit.
Steel frames: when one beam will not do
Some extensions ask more of the structure than a single beam can give. A wrap-around extension, a fully glazed rear wall, two corners removed, or an opening wider than masonry and a lone beam can handle all point to a frame.
A steel frame is a set of beams and columns connected to act together. It can carry the loads where there is no wall left to sit on, hold up the storey above an open corner, and make large unbroken glazing possible. Because the members work as a system, the engineer designs the whole frame, not just an individual beam. This is the same structural steelwork principle used in larger buildings, scaled to a home.
Supply-only versus supply-and-fit
There are two ways to buy the steel for an extension, and it helps to know the difference when you compare quotes.
- Supply only is the steel itself: beams or a frame, cut and drilled to your engineer’s specification, ready for your own builder to install.
- Supply and fit adds the installation: temporary propping, lifting the steel into place, building bearings and connections, and leaving it ready for the build to continue.
Fitting load-bearing steel is structural work. It needs propping while the wall is open and Building Control sign-off, so most homeowners use a builder or an erection team rather than fitting it themselves. At TC Rowan we do either: our own Banbury workshop and erection team can take a job from drawing to final bolt, or we supply the steel for your builder to install.
The process: calculation, fabrication, install
Every steel-framed extension follows the same broad path, and the order matters.
- Structural engineer’s calculation. Nothing is fabricated until an engineer has calculated the loads and specified the beams or frame. This is not optional for load-bearing work, and it is what makes a quote a real price rather than a guess.
- Fabrication. We work to the engineer’s drawings, cutting, drilling and welding the steel to specification in our workshop. We are CE approved and work to BS EN 1090 execution standards, with material traceability available on request.
- Install. The steel is delivered and erected: propping the structure, lifting the beams or frame into place, and making the connections and bearings. Building Control inspects the load-bearing work and signs it off.
A quick note on permissions, because it trips people up. Building Control approval is needed for the structural work whatever else applies. Separately, some extensions are permitted development and some need full planning permission, so confirm your own case with your local planning authority before you commit. We can supply or fit the steel either way; we do not make the planning decision for you.
Frequently asked questions
Do I need steel for a house extension?
Most extensions need some steel. Even a modest single-storey extension usually needs a beam (an RSJ) where you open up an existing wall or carry a load over wide doors. Larger openings, wrap-around extensions and big glazing often need a steel frame because masonry cannot span the distance. A structural engineer decides what is required and sizes it.
What is the difference between an RSJ and a steel frame for an extension?
An RSJ is a single beam that carries the load over one opening, such as a knocked-through wall or a run of bi-fold doors. A steel frame is a set of beams and columns working together, used when one beam cannot do the job: large glazed openings, two corners removed, or a wrap-around extension. Many extensions use a few beams; bigger or more open designs use a frame.
Does a steel framed extension need planning permission and Building Control?
Building Control approval is needed for the structural work whether or not planning permission applies. Some extensions fall under permitted development and some need full planning permission, so confirm your own situation with your local planning authority before you start. The steelwork itself is sized by a structural engineer’s calculation and signed off through Building Control.
Can you just supply the steel, or do you fit it too?
Both. We can supply beams and frames cut and drilled to your engineer’s specification for your builder to install, or supply and fit with our own erection team. Fitting load-bearing steel means temporary propping and Building Control sign-off, so most homeowners use a builder or our team. Tell us your situation and we will get you a free quote for supply-only or supply-and-fit.