Knock-Through & Steel Beam (RSJ) Cost London (2026 Guide)
•8 min read
A knock-through in London typically costs £800 to £2,000 for a simple opening in a non-structural wall, and £1,800 to £4,500 or more for a structural knock-through requiring a steel beam (RSJ), depending on the span and the load being carried. The single biggest factor in that range is whether the wall is load-bearing, which determines whether the job is a straightforward opening or a properly engineered structural alteration with building control sign-off. This guide covers realistic 2026 cost bands and the process a structural knock-through has to follow.
Knock-through cost in London: simple opening vs structural
A simple opening formed in a non-structural (stud or lightweight partition) wall, without any load to redirect, typically costs £800 to £2,000. This covers forming the opening, trimming it out, and making good the surrounding wall, floor and ceiling, and is a relatively contained job because there's no beam to specify, no load calculation to produce, and usually a shorter building control process, if one applies at all.
A structural knock-through, removing or opening up a load-bearing wall and installing a steel beam (RSJ) to carry the load previously carried by the wall, typically costs £1,800 to £4,500 or more. The wide range reflects how much the span and load vary between properties: a narrow opening in a lightly loaded internal wall sits toward the lower end, while an opening spanning most of a room's width, carrying the load of a full upper floor and roof, needs a larger, heavier beam and more substantial padstones or foundations to bear it, pushing the cost toward the upper end or beyond.
Knock-through and steel beam (RSJ) cost guide (London, 2026)
Typical London market range for guidance only, not a fixed Lian Construction quote. A structural engineer's assessment is required before a load-bearing wall is opened up, and this determines the final specification and cost.
Why a structural engineer's calculation comes first
Before any load-bearing wall is opened up, a structural engineer needs to calculate the size and specification of beam required to safely carry the load that wall was previously carrying, based on the span of the opening, the loads from the floors and roof above, and the bearing capacity of the walls either side where the new beam will sit. This calculation isn't a formality, it's what building control will check the installed beam against, and it's also what a steel fabricator uses to cut and supply the correct beam for the job.
Skipping this step, or using a generic 'standard size' beam without a calculation specific to the property, is one of the more serious mistakes possible in a knock-through, since an undersized or incorrectly positioned beam is a genuine structural safety issue, not just a building control technicality. Every structural knock-through we scope starts with either our own structural engineer or one of the client's choosing producing that calculation before any opening work is priced in detail.
Building control sign-off
Removing or altering a load-bearing wall falls under Part A (structure) of the building regulations, and needs either a building notice or full plans application submitted to building control before work starts, along with the structural engineer's calculation. Building control will inspect the work at the point the opening has been formed and the beam is in place but still visible, before it's covered by plaster or plasterboard, to confirm it matches the calculation and has been installed correctly.
As with any structural work, having a completion certificate on file matters at the point of a future sale. Conveyancing solicitors routinely ask whether structural alterations, including knock-throughs, had building control sign-off, and a missing certificate for a visibly altered wall is a common cause of delay or reduced offers when a property with an unregistered knock-through comes to be sold.
The typical process, step by step
A structural knock-through follows a fairly consistent sequence in London properties. First, a structural engineer surveys the wall and produces a calculation specifying the beam size, type and bearing requirements. Second, that calculation, along with a building notice or full plans application, goes to building control, which needs to be reviewed before work starts on site. Third, the steel beam itself is fabricated to the specified size and delivered to site, which can take one to two weeks depending on the fabricator's workload and whether it's a standard section or a bespoke size.
On site, temporary propping, Acrow props and timber needling, or a proprietary support system, is installed to carry the load of the wall and floors above before the wall itself is touched. The opening is then formed, the beam lifted into position and packed and bedded onto padstones or the existing wall structure, and the temporary propping removed once the beam is confirmed to be correctly bearing the load. Building control carries out its inspection at this stage, before the beam is boxed in with plasterboard and the opening is made good, plastered and decorated.
How long does a knock-through take?
A simple non-structural opening typically takes two to four days on site, covering forming the opening and making good. A structural knock-through with a steel beam typically takes one to two weeks on site once the beam has been delivered, though the structural engineer's calculation and building control process ahead of that can add two to four weeks before any on-site work starts, and beam fabrication and delivery can often run in parallel with the building control review to avoid adding extra time on top.
The most common London use case: kitchen-diner knock-throughs
The most common structural knock-through in London's Victorian and Edwardian terraces is opening the wall between the kitchen and dining room, or between the through-lounge and rear reception room, to create a single open-plan kitchen-diner or living-dining space. These walls are very often load-bearing in period terraces, since they typically carry the joists for the floor above, which is exactly why a steel beam is usually required rather than a simple opening.
Our property refurbishment London team regularly scopes this exact knock-through as part of a wider kitchen or ground-floor refurbishment, coordinating the structural work with the kitchen fit-out that typically follows it, so the beam, the finished ceiling line and the new kitchen layout are all designed together rather than the structural work being treated as a separate project.
Party wall implications if the wall is shared
Where the wall being opened up is a party wall, shared with a neighbouring property rather than an internal wall within your own home, the work is likely to fall under the Party Wall etc. Act 1996, requiring a party wall notice served on the neighbouring owner before work starts. This applies most often to knock-throughs in converted flats or maisonettes where a wall between properties is being altered, rather than the more common kitchen-diner knock-through on an internal wall within a single house, but it's worth checking early, since the party wall process typically adds several weeks to a project timeline where it applies. Our construction company London team can flag whether a specific knock-through is likely to involve a party wall during the initial survey.
Questions
Frequently asked questions
How much does a knock-through cost in London?
A simple opening in a non-structural wall typically costs £800 to £2,000. A structural knock-through requiring a steel beam (RSJ) typically costs £1,800 to £4,500 or more, depending on the span and load being carried.
How do I know if my wall is load-bearing?
There's no reliable way to tell just by looking, tapping or checking wall thickness alone. A structural engineer needs to assess the wall against the building's floor and roof structure to confirm whether it's load-bearing before any opening work is scoped or priced.
Do I need planning permission for a knock-through?
Usually not, since an internal knock-through doesn't change the external appearance of the property. Building regulations approval under Part A does apply to any structural wall alteration, regardless of whether planning permission is needed.
How long does it take to get a steel beam fabricated and delivered?
Typically one to two weeks, depending on the fabricator's workload and whether the beam is a standard section or a bespoke size calculated for the specific opening.
What size steel beam do I need for a knock-through?
This can't be generalised. Beam size depends on the span of the opening, the load from the floors and roof above, and the bearing capacity of the walls either side, all of which a structural engineer calculates for the specific property.
Will I need a party wall agreement for a knock-through?
Only if the wall being opened up is shared with a neighbouring property, which is most common in converted flats or maisonettes. A knock-through on an internal wall within a single house typically doesn't trigger the Party Wall etc. Act 1996.
Can I live in the property while a knock-through is being done?
Often yes for a single knock-through, since the work is usually contained to one area, though expect noise, dust and a period without full use of the affected rooms while propping is in place and the beam is installed.
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