Knock Through Cost in London: RSJ & Steel Beam Price Guide 2026
Updated 12 June 2026|8 min read
Knocking through an internal wall in London costs £1,800–£4,500 for a standard job, rising to £7,000 or more for complex structural openings. A simple non-structural opening can be £800–£1,200, while a load-bearing knock through with a steel beam runs from around £1,000 for a small cavity opening to £2,000 for a wide span. The cost is built from the steel, structural engineer's calculations, padstones, labour, building control and making good. This guide breaks it all down so you can budget before booking a free survey.
How much does a knock through cost in London?
An internal wall removal in London typically costs £1,800–£4,500, and complex jobs with wide spans or tricky access exceed £7,000. Where the wall is not load-bearing, a simple opening can be done for £800–£1,200. Where it is load-bearing, the price depends almost entirely on the steel beam and the span.
For a load-bearing knock through, expect around £1,000–£1,600 for a solid-brick Victorian wall, £800–£1,200 for a simple cavity opening, and £1,400–£2,000 for a large opening or a span sized for bi-fold doors. These figures cover the structural work itself; making good, decoration and any services moved are additional.
London carries a 25–40% premium over national rates. A big hidden cost in period terraces is that steel beams often cannot be craned or wheeled in and must be carried through the house by hand, which adds labour. The table below shows typical structural costs by wall type.
Type of knock through
Typical London cost (2026)
Non-structural / stud wall opening
£800 – £1,200
Simple cavity wall opening (with steel)
£800 – £1,200
Solid-brick Victorian wall opening
£1,000 – £1,600
Large opening / bi-fold span
£1,400 – £2,000
Complex multi-beam or full wall removal
£4,500 – £7,000+
What goes into the price of a knock through
The headline cost of a knock through is the sum of several distinct items, and a good quote shows each of them. First is the steel itself, the RSJ or universal beam, sized by the engineer to the load and span. Larger spans need heavier sections, which cost more and need more lifting.
Second is the structural engineer's calculations, typically £400–£900, which size the beam, specify the padstones it bears on and confirm the temporary propping needed. Third are the padstones, the concrete or steel bearings that spread the beam's load into the wall. Fourth is labour: propping the structure, cutting the opening, installing the beam, bedding it and removing the masonry safely.
Then come building control fees, around £300–£700, and finally making good, the plastering, decoration and finishing that turn a structural opening back into a finished room. Skip the engineer or building control and you are not saving money, you are storing up a problem for resale.
RSJ and steel beam installation costs
The steel beam is the heart of a structural knock through, and its cost scales with span and load. A modest RSJ for a typical terrace opening might cost £150–£400 for the steel alone, while a heavy beam for a wide kitchen-diner or a span carrying a wall above can be £600–£1,500.
Installation is where the real labour sits. The wall above must be propped with acrow props and strongboys before any masonry is cut, the opening is formed in stages, the beam is lifted into place, often by hand in a terrace, and it is bedded onto padstones and pinned tightly to the masonry above so nothing settles.
For a single-beam load-bearing opening, the all-in structural cost of supplying and installing the steel, including engineer and building control, generally lands in the £1,000–£2,000 range we set out above. Wide openings needing two beams bolted together, or a goalpost frame with vertical posts, push the cost higher.
Why London terraces cost more
London's housing stock makes knock throughs more involved than the same job elsewhere. Victorian and Edwardian terraces have solid one-brick walls rather than modern cavity construction, so there is more masonry to remove and the loads are heavier, needing bigger beams.
Access is the other big factor. In a mid-terrace there is no side return to wheel materials through and no space to crane a beam in over the roof, so a heavy steel often has to be carried in by hand through the front door and the hallway. That takes more people and more time, and labour is the largest part of any London building cost.
Parking suspensions for the skip and deliveries, restricted working hours in some boroughs, and the need to protect shared party walls all add to the bill. None of this is padding, it is the genuine cost of doing structural work properly in central London, and we build it into the quote rather than springing it on you mid-job.
Building control and structural sign-off
Removing a load-bearing wall is notifiable structural work, so it must go through building control, either via your local authority or an approved inspector. The fee in London typically runs £300–£700, and the process protects you.
Building control reviews the engineer's calculations, inspects the steelwork and bearings before they are plastered over, and issues a completion certificate. That certificate is essential at resale: a buyer's surveyor will spot an opening where a wall once stood and ask for proof it was done with proper consent and support. Undocumented knock throughs are a common reason sales stall.
We arrange the building control notification, coordinate the structural engineer, and book the inspections as part of the works. You receive the completion certificate on finishing, so your paperwork is clean for the day you come to sell.
Don't forget making good and the extras
The structural opening is only part of the project. Once the beam is in, the room has to be finished, and the extras are where DIY-style quotes fall short. Making good covers plastering the new reveals and the beam, often boxing the steel in with plasterboard, patching the ceiling, fitting new skirting and flooring across the old wall line, and redecorating both spaces.
If the wall carried a radiator, switches or sockets, those services need rerouting by the relevant trade. Opening two rooms into one also changes the heating and sometimes the lighting layout, which may add electrical and plumbing work.
Making good typically adds £600–£2,000 depending on the size of the opening and the finish you want. We price knock throughs to include making good back to a paint-ready or fully decorated standard, and we set out clearly in the quote what is included so the finished result is a proper room, not a hole with a beam in it.
Get a fixed quote for your knock through
Because a knock through depends on the wall type, the span, the load above and the access, an accurate price needs a survey, not a number over the phone. A builder who quotes firm without seeing the wall is guessing, and that guess becomes an extra later.
A proper quote names the beam the engineer specifies, lists who arranges the structural calculations and building control, accounts for any services to be moved, states the making-good finish included and shows VAT separately. It should also be honest about access in a terrace, because that drives the labour.
At London Refurbishments & Leak Repairs we provide a free survey and a fixed written quote covering the steel, the engineer, building control, the structural work and making good. You get one accountable price for opening up your space properly.
Frequently Asked Questions
How much does a knock through cost in London?
A standard internal wall knock through in London costs £1,800–£4,500 in 2026, with complex openings exceeding £7,000. A non-structural opening can be £800–£1,200, while a load-bearing opening with a steel beam runs £1,000–£2,000 for the structural work. Making good and VAT are additional.
How much does it cost to install an RSJ steel beam in London?
Supplying and installing an RSJ steel beam for a typical London knock through costs around £1,000–£2,000 all in, including the engineer's calculations and building control. The steel alone is £150–£1,500 depending on span and load, with the rest covering propping, installation, padstones and labour. Wide or multi-beam spans cost more.
Do I need building control for a knock through?
Yes. Removing a load-bearing wall is notifiable structural work, so it must go through building control via your local authority or an approved inspector, with fees around £300–£700 in London. Building control inspects the steelwork before it is covered and issues a completion certificate you will need at resale.
Why is a knock through more expensive in a Victorian terrace?
Victorian terraces have solid one-brick walls rather than cavity construction, so there is more masonry to remove and heavier loads needing larger beams. Mid-terrace access often means carrying the steel in by hand through the house, which adds labour. These factors push the cost toward the upper end of the range.
How long does a knock through take?
A standard load-bearing knock through usually takes three to five days for the structural work, plus making good and decoration. Allow extra lead time beforehand for the structural engineer's calculations and building control notification, which should be in place before any wall is opened up.
