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Knocking two rooms into one open-plan kitchen-diner is the single most requested alteration we are asked about in North London. Done properly it is a straightforward, well-understood piece of structural work. Done without calculations, propping or Building Control it is dangerous, and it will cost you far more to put right — or to explain to a buyer's solicitor years later.
How to tell if a wall is load-bearing
There are reliable indicators you can check yourself before anyone visits, though none of them is a substitute for a structural engineer's assessment.
- Direction of the floor joists above: joists that run into the wall, rather than parallel with it, suggest the wall is carrying them.
- Thickness and material: solid masonry or blockwork around 100mm or more is far more likely to be structural than a hollow-sounding timber stud partition.
- What sits directly above: a wall that lines up with another wall on the floor above, or with a roof purlin or a chimney breast, is almost certainly load-bearing.
- Position in the plan: internal spine walls running through the middle of a typical Victorian or 1930s house usually carry both floor and roof loads.
- Age and alterations: previous owners' work can transfer loads in unexpected ways, so assumptions based on layout alone are unreliable.
A structural engineer site visit is essential before any demolition begins. The engineer confirms what the wall carries, calculates the beam size, and specifies the padstones or spreader plates the beam bears onto. Removing masonry first and asking questions later is how floors above end up cracking and sagging — and the remedial work costs multiples of the calculations you skipped.
Cost breakdown: what you are actually paying for
The ranges below are indicative guide figures for typical UK domestic projects, weighted towards North London and Hertfordshire where labour and access costs sit above the national average. They exclude VAT and professional fees, and they are not a quotation — an accurate price follows a site visit and the engineer's calculations.
| Item | Indicative range |
|---|---|
| Small opening / single doorway with lintel | £1,200 – £2,200 |
| Standard knock-through (approx. 3m RSJ) | £2,000 – £3,800 |
| Large opening / double steel or goalpost frame (4m+) | £4,500 – £7,500+ |
| Structural engineer calculations | £400 – £850 |
| Building Control notice and inspections | £250 – £500 |
| Plastering, first-fix electrics and making good | £800 – £1,800 |
Two things push a price up more than anything else: span and load path. A 2.4m opening under a single floor is routine. A 5m opening carrying the roof, where the steel has to be craned or cut in sections and bolted, is a different job with different propping, different padstones and often a steel-to-steel connection designed by the engineer.
Get a ballpark range for your opening
Guide figures only, excluding VAT. An accurate price follows a site visit and the structural engineer's calculations.
Planning this as part of a wider project? Read our house extension cost guide — a knock-through is often cheaper when carried out alongside extension works rather than as a standalone job. And before you appoint anyone, our guide to 10 things to look for when choosing a builder covers what to check.
The 4-step knock-through process
Step 1: Structural engineer calculations and padstones
The engineer measures the span, establishes what the wall carries, and issues calculations specifying the steel section (for example a 203x133 UB), the bearing length at each end, and the concrete padstones or steel spreader plates that transfer the load down into the remaining masonry. These calculations are the document Building Control assesses.
Step 2: Building Control notice submission
Removing a load-bearing wall is notifiable work under the Building Regulations. You submit either a Building Notice or a Full Plans application to your local authority, or appoint an Approved Inspector. Structure, fire resistance and any affected means of escape are all inspected.
Step 3: Temporary propping
Before a single brick is removed, the load above is picked up on Acrow props with Strongboys or needle beams, spread across sole plates and head plates so the props do not punch through the floor. Propping is set out to the engineer's requirements and stays in place until the new beam is fully bedded and loaded.
Step 4: Demolition, RSJ installation and fire protection
The masonry comes out in a controlled sequence, the padstones are bedded, the beam is lifted into position and packed and pinned so it takes the load evenly, and the bearings are built up solid. The steel is then encased for fire resistance — typically pink fireline plasterboard to achieve the required period — before plastering, first-fix electrics and making good.
Does it need a party wall agreement?
If either end of the new beam bears onto a wall shared with a neighbour — a terraced or semi-detached party wall — the work falls under the Party Wall etc. Act 1996. Cutting into a party wall to form a bearing, or inserting a beam supported by it, requires formal written notice to the adjoining owner, normally two months before work starts.
Notice is not a request for permission, but ignoring it removes your legal protection if the neighbour later claims damage. Where a neighbour dissents, surveyors are appointed and an award is drawn up setting out how the work proceeds and recording the condition of their property beforehand.
Our party wall guide for North London explains notices, dissent and surveyor costs in more detail.
Building Control sign-off and the completion certificate
Once the beam is in, encased and inspected, Building Control issues a completion certificate. That single document is what proves the structural alteration was carried out and approved properly.
It matters far beyond the build. When you sell, your buyer's solicitor will ask for certificates covering any structural work. Without one, the sale stalls: buyers request a retrospective regularisation application, an indemnity policy, or a price reduction — and a regularisation can mean opening up finished plasterwork so an inspector can see the steel. Unsigned work is a liability that follows the house.
Frequently asked questions
Can I remove a load-bearing wall myself?
It is not illegal to carry out the work yourself, but it is notifiable under the Building Regulations and it must be designed by a structural engineer, propped correctly and inspected. In practice the propping and beam installation is specialist work, and a failure risks collapse of the floor above. You would also still need the completion certificate to sell the property.
How long does installing an RSJ take?
A straightforward single knock-through with a 3m beam is typically 3 to 5 working days on site for the structural element, plus plastering and making good. Larger openings, goalpost frames or beams carrying the roof usually run 1 to 2 weeks. Allow additional lead time beforehand for engineer's calculations, steel fabrication and the Building Control notice.
Do I need planning permission for an internal wall?
Not usually — internal alterations do not normally need planning permission because they do not change the external appearance of the building. Building Regulations approval is still required. The exceptions are listed buildings, where listed building consent is needed for internal changes, and some flats or properties covered by an Article 4 direction or leasehold conditions.
Planning an open-plan knock-through in North London? Speak to The Construction Collective team for structural calculations, steel installation, and Building Control sign-off.
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