Budget
£3,050
- Focused essentials
- Practical finishes
Open Plan Knock Through Costs in London
Estimates derived from UK trade benchmark data and regional labour indices, updated May 2026. Methodology →
Open Plan Knock Through in London typically lands above the UK-wide average for the same spec. We start from our national guide ranges and reflect the labour and logistics pressure you usually see in the capital.
In London, labour and logistics costs are typically highest across UK regions. For the full UK-wide baseline, compare with Open-Plan Knock-Through Cost UK.
Two ways to take action on knock-through costs
Pick the path that fits where you are — running early numbers, or pressure-testing a quote you've already got.
Typical London open plan knock through budgets
Three planning tiers for open plan knock through in London, with scope and a representative figure for each. Run your own numbers in the calculator for a tailored range.
Mid-rangeMost common
£5,500
- Balanced specification with core upgrades
- Reliable materials
Premium
£11,000
- Premium materials
- Wider scope with higher coordination demands
Typical regional cost ranges
| Item | Cost Range |
|---|---|
| Non-load-bearing wall removal | £600 – £1,850 |
| Small steel beam (up to 3m) | £1,850 – £4,250 |
| Medium beam (3–4m) | £3,050 – £6,700 |
| Large opening (4m+) | £4,900 – £12,200 |
| Party wall agreement | £600 – £2,450 |
Mini knock-through cost calculator
Three quick inputs and we'll email you an indicative range. Run the full calculator for a postcode-adjusted estimate.
What's included in London open plan knock through costs
- Load-bearing vs partition — load-bearing walls need a steel and building regs.
- Span — longer openings need bigger (more expensive) beams.
- Party wall — terraced and semi-detached may need party wall agreement.
- Access — getting the steel into the property can add cost.
- Making good — plastering, flooring, and decoration after the opening.
- Location — London and the South East typically cost 15–25% more.
5 line items every fair knock-through quote should include
Use this checklist to spot missing scope before you sign — each item names what should be priced and what to ask for if it isn't.
- 1
Structural engineer calculations + Building Regs application
Removing or modifying a load-bearing wall is regulated structural work. You need an IStructE-registered engineer to calculate the steel beam (RSJ) size and pad-stones required to support the loads, and a Building Regs application (Full Plans or Building Notice). Without these, the work is illegal and dangerous.
Fair UK range: £500-£1,200 for structural engineer fees on a typical knock-through; £200-£500 Building Control fees.
Ask: Who's the structural engineer, are they IStructE-registered, and is Building Regs Full Plans application included?
- 2
RSJ (steel beam) supply + installation
The steel beam (typically 178x102 UB to 305x165 UB depending on span and load) supports everything above the new opening. Cost varies by size, length, and lifting access. Should include: steel itself, pad-stones (concrete blocks supporting beam ends), steel painting/fire protection.
Fair UK range: £800-£2,500 for typical RSJ + pad-stones on a 3m opening; significantly more for longer spans.
Ask: What size is the RSJ and what loads does it carry? Is fire protection (typically 2-coat intumescent paint or boxing) included?
- 3
Temporary propping during work
Before removing the wall, the building must be temporarily propped (Acrow props, needle beams) to support loads while the RSJ is installed. This is critical safety work, not optional. A quote that doesn't mention propping is dangerous.
Fair UK range: £300-£800 for propping on a typical knock-through.
Ask: Is temporary propping included, and what's the propping plan during RSJ installation?
- 4
Reinstatement — plaster, decoration, flooring, services relocation
After the wall comes down: plaster repair to ceiling and remaining walls, redecorate, repair/replace flooring across the new opening (no longer separated), relocate any services that were in the wall (sockets, light switches, radiator, sometimes plumbing).
Fair UK range: £1,500-£4,000 for full reinstatement on a typical knock-through.
Ask: Is reinstatement (plaster + decoration + flooring + services) itemised separately?
- 5
Party Wall Award (if shared with neighbour)
On a terraced or semi-detached house, knock-throughs that affect a wall shared with a neighbour (party wall) trigger the Party Wall etc. Act 1996. Surveyors must be appointed (typically one each side, or one shared). This takes 4-8 weeks and adds £700-£1,500 to costs.
Fair UK range: £700-£1,500 in party wall surveyor fees; £0 if no shared wall affected.
Ask: Does this knock-through affect any party walls, and have you allowed for surveyor fees and timing?
Want this run on your actual knock-through quote? Upload it and our AI Quote Checker flags missing line items, overcharges and the questions worth asking.
7 red flags that mean you might be overcharged on a knock-through quote
UK-specific signals — each red flag explains why it matters and the question that surfaces the truth.
No structural engineer named on the quote
Why it matters: Removing or modifying load-bearing walls without IStructE-registered structural calculations is dangerous and illegal. Without proper calcs, the RSJ may be undersized, leading to cracks, sagging, or structural failure over years.
Ask: Who is the structural engineer, are they IStructE-registered, and can you share their PI insurance details?
No Building Regs Full Plans application
Why it matters: Structural changes need Building Regs sign-off (Full Plans preferred over Building Notice for structural work). Without it, the work is illegal, can't be insured against future failure, and you'll struggle to sell the house.
Ask: Will you submit Building Regs as Full Plans (with engineer's calcs), and arrange Building Control inspections at key stages?
No mention of Party Wall Act for attached property
Why it matters: On a terraced or semi-detached house, knock-throughs near or affecting party walls trigger the Party Wall Act 1996. Without proper notice and award, you face injunctions, neighbour disputes, and forced rectification work.
Ask: Will this trigger the Party Wall Act? If so, have you allowed for surveyor fees and 4-8 week timing?
No temporary propping plan described
Why it matters: Wall removal without proper propping causes structural collapse. Reputable contractors explain the propping plan: where Acrows go, how the loads are supported during RSJ install, sequence of operations.
Ask: Show me the propping plan — where do Acrow props go, and what's the sequence for installing the RSJ?
Single quote with no separate structural engineer fee
Why it matters: Structural engineer fees should be a separate line item. If they're bundled into a 'structural works' total, the contractor is either using an unqualified engineer or skipping engineering entirely. Either way, dangerous.
Ask: Is the structural engineer fee a separate line item, and can I see the engineer's calculations?
Quote significantly below £2,500 for a 3m+ load-bearing knock-through
Why it matters: UK 2026 typical for a 3m+ load-bearing knock-through is £4,000-£8,000 all-in (engineer + Building Regs + RSJ + reinstatement). Below £2,500 usually means: no engineer, no Building Regs, no propping, basic reinstatement. The work will be unsafe.
Ask: Can you walk me through how you've achieved this price? What's included for engineer, Building Regs, propping, and reinstatement?
No fire protection mentioned for the RSJ
Why it matters: Building Regs Part B (fire) requires steel beams in domestic situations to be fire-protected (typically 30 or 60 minute rating). This is usually 2 coats of intumescent paint or fire-rated boxing. A quote without fire protection will fail Building Control.
Ask: What fire protection is on the RSJ — intumescent paint, boxing, or board-clad? What rating is specified?
Spot a couple of these on your knock-through quote? Upload it for a full red-flag scan and fair-rate comparison.
How to negotiate a knock-through quote
A simple framework, a verbatim script you can paste into an email or text, and the topic-specific levers that move the price.
Framework
- 1Get three quotes from FMB-registered or TrustMark-accredited contractors who can demonstrate structural work experience. Each must visit the property and quote on identical scope: same opening width, same wall type, same services to relocate, same reinstatement level.
- 2Demand itemised quotes covering: structural engineer fee, Building Regs fee, RSJ supply + install, propping, fire protection, services relocation, plaster + decoration, flooring continuity, party wall fees (if applicable). Reject single-total quotes — too easy to skip engineering or fire protection.
- 3Identify the median per major line. The total spread on knock-throughs is often £2-5k across three quotes — meaningful. The line-item spread shows you who's lowballing the engineer fee or padding the structural work.
- 4Insist on a written agreement (JCT Minor Works for £10k+ projects) defining payment schedule (stage payments tied to milestones — engineer's report received, RSJ in place, reinstatement complete), variations process, and warranty terms.
Verbatim script
I've had three quotes for this knock-through. Yours is competitive overall, but the structural engineer fee is £X above the median I've received from two other FMB-registered contractors, and the RSJ + install line is £Y above. The other quotes specify [RSJ size] with engineer's calcs from [IStructE engineer]. Can you walk me through your engineering and structural pricing, confirm the spec is comparable, and is fire protection included to a 30-minute rating?
Topic-specific levers
- Steel-supply only: source the RSJ direct from a local steel fabricator (£200-£500 saving on £800 RSJ) and pay the contractor for fitting only. Requires you to coordinate delivery and lifting.
- Combine multiple knock-throughs: if doing 2 openings at once, the engineer fee, Building Regs application, and propping infrastructure are shared — saves £500-£1,500.
- Decoration self-supply: do the decoration yourself after structural work complete. Save £400-£1,000.
- Time-of-year scheduling: structural builders are quieter October-February; bookings then often save 10-15% vs spring/summer.
- Bundle with kitchen renovation: if you're renovating the kitchen anyway, having the knock-through done as part of the same project saves on contractor mobilisation and reinstatement coordination.
Want to know which line items on your knock-through quote are above market before you negotiate? Upload it for a fair-rate comparison.
10 questions to ask before hiring a structural builder
Vet on competence, insurance, paperwork and process — not price alone. Each question spells out the answer you want and why.
1. Are you a member of the FMB (Federation of Master Builders) or TrustMark?
Why it matters: FMB and TrustMark vet contractors on workmanship and finances. Both offer Insurance-Backed Warranties for structural work.
2. Can you show me 2-3 completed knock-throughs from the last 12 months in similar property types?
Why it matters: Knock-through experience matters more than general building experience. Recent local references let you visit completed openings and ask homeowners about post-work cracks, sagging, or issues.
3. Who's your structural engineer, and are they IStructE-registered?
Why it matters: Structural engineering is critical and regulated. The engineer should be IStructE-registered with current PI insurance. Vague 'we'll get one' usually means they don't have an established relationship.
4. Will you handle Building Regs Full Plans application and arrange inspections?
Why it matters: Full Plans submission gives you written approval before work starts. Some contractors prefer Building Notice (less paperwork, more risk for you on a structural job).
5. What's the temporary propping plan during RSJ installation?
Why it matters: Reputable contractors have a clear propping methodology: Acrow positions, sequence, how loads transfer during the work. Vague answers indicate inexperience with structural work.
6. Is fire protection of the RSJ included, and to what rating?
Why it matters: Building Regs Part B requires steel fire protection in domestic situations. 30-minute is typical for ground-floor knock-throughs; 60-minute may be needed in certain configurations. Without spec, the work fails Building Control.
7. What's your payment schedule, and what milestones trigger each stage?
Why it matters: Stage payments tied to verifiable milestones (engineer's report received, Building Regs approval, RSJ installed, reinstatement complete) protect you. Industry norm: max 25% upfront.
8. Are you carrying insurance for this work, and what's the cover?
Why it matters: Structural work is high-risk. Public liability of £5M minimum is industry norm. Professional indemnity insurance (separate from public liability) covers design-related claims. Ask to see both certificates.
9. What's the warranty on the workmanship, and is it insurance-backed?
Why it matters: Industry norm: 10-year insurance-backed structural warranty (FMB IBG, BuildSure, etc.) for structural work. Verbal-only warranties are worthless.
10. Are you VAT registered, and will you provide a proper invoice?
Why it matters: VAT registration matters for invoice and warranty enforcement. Cash-only or no-invoice arrangements forfeit consumer protection on a £5-15k structural job.
Already chosen a structural builder and got a quote? Run it through our Quote Checker before you commit.
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