Knocking through a wall to create a large open-plan kitchen and diner is one of the most popular renovations in the UK. However, if that wall is load-bearing, you cannot just take a sledgehammer to it. You have to install a Rolled Steel Joist (RSJ) to hold up the ceiling and roof above it.
I have managed countless knock-throughs in my property career, and I always see homeowners underestimating the true budget required. They budget for the steel beam but completely forget about the structural engineer, the local council fees, and the massive amount of dust and rubble. Here is the realistic cost breakdown for installing an RSJ so you do not get caught out.
How much does it cost to install an RSJ? In the UK, the average cost to supply and install a standard RSJ support beam is between £1,500 and £2,500. This includes the steel beam (£200 to £500), builder labour (£800 to £1,200), structural engineer calculations (£300), and Building Control approval fees (£200). Larger spans or hidden costs like rerouting plumbing can push the final bill over £3,500.
Average RSJ Installation Costs (£)
To give you a realistic idea of what you will spend, I have broken down the average costs for a standard UK structural alteration. These prices assume straightforward access and a standard brick or block property.
| Item / Service | Average UK Cost |
| RSJ Steel Beam (2m to 4m) | £200 to £500 |
| Builder Labour (2 to 3 days) | £800 to £1,200 |
| Structural Engineer Calculations | £250 to £400 |
| Building Control Fees | £200 to £300 |
| Skip Hire (for brick rubble) | £250 to £350 |
| Total Estimated Cost | £1,700 to £2,750 |
Structural Engineers and Building Control Fees
Before a builder even touches your property, you have to spend money on paperwork and calculations. This is a strict legal requirement in the UK.
First, you need a structural engineer. They will visit your home, inspect the load above the wall, and calculate the exact size, weight, and specification of the RSJ needed. They also calculate the size of the concrete padstones the beam will sit on. You can find a qualified professional through the Institution of Structural Engineers. Expect to pay between £250 and £400 for these calculations.
Once you have those calculations, you must submit a Building Notice to your local council Building Control department before work starts. A building inspector will come out to check the beam once it is resting on its padstones and sign it off as structurally safe. Building Control fees vary by council but usually cost between £200 and £300.
The Hidden Costs of Knocking Down a Wall
The steel beam and the builder are only half the story. Taking out a wall creates a domino effect of other trades and materials you need to pay for.
- Rerouting Services: Internal walls are rarely empty. They often contain light switches, plug sockets, or radiator pipes. You will need to hire a plumber or electrician to reroute these services before the wall comes down.
- Waste Removal: Bricks, mortar, and plaster weigh an absolute ton. A standard knock-through will easily fill a skip. Read our guide on what size skip you need to handle heavy masonry waste without overloading it.
- Making Good: Once the beam is in and signed off, it has to be boxed in with fire-rated plasterboard and skimmed by a plasterer. You will then need to paint the new space and feather in the flooring where the old wall used to sit. If you want to try and smooth walls without plastering to save money, be aware that structural boxing-in usually requires a professional skim coat for a seamless finish.
Do You Need a Party Wall Agreement?
This is the biggest trap I see homeowners fall into. If your house is a terraced or semi-detached property, and the RSJ needs to rest inside a wall you share with your neighbour, the Party Wall Act 1996 applies.
You are legally required to serve a Party Wall Notice to your neighbour detailing the structural work. If they consent in writing, you only pay a small admin fee. If they dispute the work or demand a professional assessment, you must hire a Party Wall Surveyor. You are liable for both your surveyor’s fees and your neighbour’s surveyor’s fees. This can easily add £500 to £1,500 to your overall budget before a single brick is removed.
Can You Install an RSJ Yourself?
Absolutely not. This is not a weekend DIY project. If you install an RSJ incorrectly, the floors above will sag or the entire roof structure could collapse.
You will not be able to get the Building Control completion certificate if you do it yourself without professional oversight. Without that certificate, your home insurance is completely invalidated and you will be legally unable to sell the house in the future. Always hire a professional builder with comprehensive liability insurance.
Once the structural work is secure and the dust has settled, you can safely move onto the fun part of the renovation. If a knock-through was the first step for a major layout change, you will need to factor in your new kitchen installation costs next.
FAQs
Do I need planning permission for an RSJ?
In most cases, you do not need planning permission for internal structural alterations like installing an RSJ, as it falls under Permitted Development. However, you absolutely must get Building Control approval. Always check the official Planning Portal guidelines if you live in a listed building or a conservation area.
How do I know if a wall is load-bearing?
Never guess based on tapping the wall. Generally, if a wall runs at a 90-degree angle to the floor joists above it, or if it sits directly above another supporting wall on the ground floor, it is load-bearing. However, you must always hire a structural engineer or an experienced builder to confirm this before starting work.
