How much does a steel beam (RSJ) cost?
Installing a steel beam for a knock-through bundles the steel, structural design, labour and making good. Get an indicative figure below.
What a knock-through involves
- 1Structural design
A structural engineer sizes the beam and padstones and provides calculations for building control.
- 2Install and support
The wall is temporarily supported (Acrow props), the beam installed onto padstones, then loads transferred.
- 3Building control and finish
Building control sign-off, then plastering and making good around the new opening.
Opening up a tired terrace into an open-plan kitchen-diner is one of the most valuable layout changes on a flip — but it's structural work, so cost it with the engineer's calcs and building control included, not just the steel.
Steel beam / knock-through costs (indicative)
| Scope | Description | Indicative cost |
|---|---|---|
| Small opening | Single beam, simple | £1,500 – £2,500 |
| Through-lounge | Larger span | £2,500 – £4,000 |
| Full-width (rear) | Big span, padstones | £4,000 – £7,000+ |
| Structural calcs only | Engineer | £300 – £700 |
Includes engineer's calcs, steel, install and make-good. Party wall and RSJ size vary by span.
Frequently asked questions
How much does a steel beam cost to install?+
A typical single knock-through with an RSJ costs £1,500–£4,000 including the engineer's calculations, steel, installation and making good. Large full-width spans cost more.
Do I need building regulations for a knock-through?+
Yes — removing a load-bearing wall needs structural calculations and building control approval. It's not a job to do without engineer sign-off.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.