Key Takeaways
- Knocking two reception rooms into one costs £2,500 to £6,500+ for the structural work, with a straightforward 3 m opening at £2,500 to £5,700 once every line item is added (Checkatrade and MyJobQuote, 2026).
- It needs a structural steel beam (RSJ), engineer's calculations at £300 to £700, and Building Control sign-off.
- No planning permission for internal work, but listed building consent applies if the house is listed.
- A party wall notice is usually needed, served two months before work starts.
- Allow 4 to 8 weeks overall; the steelwork itself is 3 to 5 days.
- Consider keeping a partial wall or wide opening for some separation.
Knocking through the two reception rooms of a London terrace into one open, light-filled space is one of the most popular renovations there is. Here is what it costs, the structural work involved, and how the process runs.
Why knock through?
Because the back reception room in a London terrace is usually the darkest room in the house. These homes were built with two separate rooms, front and back, split by a masonry wall, and the rear one gets its light from a single window facing a narrow garden. Take the wall out and the front bay lights the whole floor. Combined with a side return extension, it is how most London terraces end up with a large open-plan kitchen-diner.
Be clear-eyed about the trade-offs first. One large room means cooking smells, television noise and homework share the same air, it is harder to heat, and you lose the option of shutting a door on a mess. Broken-plan, with partial dividers instead of a full wall, is the middle route.
Why it needs steel
That dividing wall is almost always load-bearing. It carries the first floor joists, often the wall above it, and in most terraces the two chimney breasts are built into it. It cannot simply be taken out: it has to be replaced with an engineered steel beam (RSJ) sized by a structural engineer and signed off by Building Control.
Two details decide how the finished room looks. A downstand beam sits below the ceiling line, the cheaper and simpler option, leaving a visible band across the room that many people box in as a feature. A flush beam is set up between the joists so the ceiling runs through uninterrupted, which costs more because the joists have to be hung off the steel. And if the masonry below the beam ends cannot take the new point loads, the engineer will specify a goalpost frame with steel posts down to pad foundations, common in terraces where the chimney breasts are also coming out.
How much does a knock-through cost?
Budget £2,500 to £6,500 or more. A single 3 m ground-floor opening with clear access lands at £2,500 to £5,700 all in, the sum of the rows below, with the lower end for a short span, a light load and a council fee at the bottom of its range; twin beams or a goalpost frame push it to £7,500. The steel is a small part: engineer, propping, building control and making good carry most of the cost. If you also want the floors levelled, services rerouted and both rooms decorated as one, that is an open-plan conversion, typically £3,000 to £8,000+.
| Item | Typical cost | Notes |
|---|---|---|
| Structural engineer's calculations | £300 to £700 | Beam sizing, bearings, padstones, building control pack |
| Steel supplied and installed | £300 to £600 per metre | Labour, lifting, padstones and fixings |
| Building control application | £300 to £900 | London boroughs sit at the top of the range |
| Propping, skip and waste | £400 to £800 | Add £30 to £80 if the skip goes on the road |
| Making good: plaster, fire boxing, decoration | £600 to £1,500 | Per opening, flooring excluded |
| Single 3 m opening, all in | £2,500 to £5,700 | Downstand beam, ground floor, clear access |
| Wide opening, twin beam or goalpost frame | £4,000 to £7,500 | Posts down to new pad foundations |
| Levelling floors across both rooms | £800 to £2,000 | Common where front and back rooms differ in height |
Do you need permission?
You need Building Regulations approval, not planning permission. Internal alterations are not development, so planning does not bite, but the beam has to be designed by a structural engineer and inspected and signed off by building control under the Building Regulations, and the finished job produces a completion certificate your buyer's solicitor will ask for. There are two exceptions worth checking: if the house is listed, internal alterations that affect its character need listed building consent, and removing a wall without it is a criminal offence; and if you are in a conservation area under an Article 4 direction, anything affecting the exterior is caught.
Building control will also look at the knock-on effects. The smoke alarm layout normally has to be brought up to current standards with mains-powered, interlinked detectors, and where an open-plan room ends up connecting directly to a stair, means of escape comes into it too.
Do you need a party wall notice?
Usually, because the beam bears into the shared wall. Cutting a pocket into a party wall to seat a steel beam is notifiable under the Party Wall etc. Act 1996, so serve a party structure notice on each affected neighbour at least two months before work starts. They have 14 days to reply, and silence counts as dissent. A written consent costs nothing; a dissent means surveyors and a party wall award, typically £900 to £2,700 with one agreed surveyor, or £1,800 to £5,400 where each side appoints its own, paid by you, plus three to six weeks. Talk to the neighbour before the formal letter arrives, not after it. Our party wall page sets out what the contractor does and does not do in that process.
Notice periods from the Party Wall etc. Act 1996 explanatory booklet (gov.uk); surveyor fees from HomeOwners Alliance, 2026.The process and how long it takes
Allow 4 to 8 weeks from first call to a finished room, longer with a party wall award. Calculations take one to two weeks, steel is fabricated within about a week of order, and the site work is 3 to 5 days for a single beam before finishes.
- Structural engineer's calculations to size the beam.
- Building Control application made, and any party wall notices served by you as building owner.
- Services traced and diverted, and the room protected.
- Temporary propping installed to both floors.
- Wall removed and the beam seated on padstones.
- Building Control inspection before the steel is concealed.
- Making good: fire boxing, plastering, flooring and decoration.
Full knock-through or partial?
You do not have to go fully open-plan, and in a period terrace there are four options worth pricing side by side.
- Full opening, wall to wall, with the chimney breasts removed too. The most open, the most expensive, the biggest change.
- Opening between the chimney breasts, leaving both breasts and their alcoves. Cheaper, keeps the storage, gives the room a natural waist.
- Twin openings either side of a retained central breast, which keeps period character.
- Wide cased opening or Crittall-style screen, connecting the rooms visually while letting you shut off sound and heat. See our Crittall doors guide.
What people regret afterwards
- Heating. Two rooms of radiators sized for two rooms rarely heat one large space. Get the heat loss recalculated before the plaster goes on, not after the first cold snap.
- Floor levels. Front and back reception rooms are often a step apart, or on different joist runs. Levelling is straightforward if it is planned, expensive if it is discovered.
- Acoustics. A long hard-floored room with no dividing wall echoes. Rugs, soft furnishing and a bit of ceiling treatment are worth budgeting for. Soundproofing the party wall is a separate job worth pricing at the same time.
- Lighting. Two ceiling roses centred on the old rooms look wrong in the new one. Plan the layout around the beam and rewire before plastering.
- Losing a second reception room. In a family house a closeable room is worth a lot. If you only have two, think hard before merging both.
Getting it done
We appoint the structural engineer, who designs and sizes the beam, manage the building control inspections, and give you the drawings and programme your party wall notice needs. You serve the notice as building owner (or your surveyor does), and we build to any award. See load-bearing wall removal in London and RSJ installation in London, or read our guides to RSJs and load-bearing walls and chimney breast removal. For a free quote, contact us or call 07472 424 226.