Steelwork Cost Estimator

Structural steelwork for a UK extension is priced per opening, not as a single lump sum. A lintel over a standard window or internal doorway is supplied and fitted for £250 – £600. A wide steel beam, often called an RSJ, spanning a bifold or sliding-door opening of roughly 2 to 4 metres runs £900 – £2,500 per opening including supply, fabrication, and installation. This estimator adds those per-opening rates together, applies an uplift for older houses, and shows a realistic total range you can check a builder's quote against.

What drives the cost

Four things move the number. The count and type of openings matters most: standard lintels sit at the low end, while a wide RSJ across a knocked-through rear wall is the single most expensive element on most extensions. Beyond the openings, two optional items add cost. A structural ridge beam or secondary roof steels add £500 – £1,200, and a cantilever or independent flat-roof steel frame adds £1,000 – £1,500. Whether you need either is a decision for your structural engineer, not the builder.

House age is the fourth factor. A modern property takes the base rates above. Interwar and Victorian houses carry an uplift because original timber lintels, unknown party wall conditions, and uncharted service routes turn a straightforward beam swap into remedial work nobody quoted for. The estimator raises every rate for those properties rather than leaving you with a figure that only holds on paper.

A worked example

Take a rear kitchen extension with one wide bifold opening and two standard lintels above the new windows. The bifold beam is priced at £900 – £2,500, and each lintel at £250 – £600. If the roof design also needs a structural ridge beam, add £500 – £1,200. The calculator sums those per-element ranges into the total shown above the fold, so you see a range rather than one misleading point figure.

Then hold a buffer. Steelwork on a real Oxfordshire kitchen extension started near its first quote and grew across several fabrication rounds once cavity widths and extra padstones came to light. That is why the estimator recommends 50% on top of the base total, and why building control checks bearing lengths and padstone positions before any steel is boxed in. See steels and lintels for how the calculation, fabrication, and installation chain fits together.