Knocking Through Two Rooms: Cost, Steel & Process (2026)

Key Takeaways

Knocking through two reception rooms is the classic terrace transformation.
It needs a structural steel beam (RSJ) and Building Control sign-off.
Typical cost is £2,500 to £6,500+ for the structural work.
A party wall agreement is often required.
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?

Many London terraces were built with two separate reception rooms, front and back, divided by a wall. Removing that wall creates a single, generous, light-filled space, often combined with a side return extension to form a large open-plan kitchen-diner. It is the defining transformation of the London terrace.

It needs steel

That dividing wall is almost always load-bearing, so it cannot simply be removed. It must be replaced with an engineered steel beam (RSJ), sized by a structural engineer and signed off by Building Control. The structural work typically costs £2,500 to £6,500+ depending on span and load.

The process

Structural engineer's calculations to size the beam.
Building Control and any party wall notices arranged.
Temporary propping installed.
Wall removed and the beam seated on padstones.
Building Control inspection before the steel is concealed.
Making good: plastering, flooring and decoration.

Full knock-through or partial?

You do not have to go fully open-plan. Keeping a wide cased opening, a partial wall, or twin openings either side of the chimney breast retains some definition and period character while still connecting the rooms. We will advise on what suits your home and how you want to live. For a free quote, contact us or call 07472 424 226.

The GS Renovation Team

GS Renovation & Home Improvements has delivered kitchen, bathroom, structural and extension projects across London for over 30 years. This guide reflects current UK industry pricing and our hands-on site experience.

Frequently Asked Questions

How much does it cost to knock two rooms into one?

Knocking through two rooms and installing the necessary steel beam typically costs £2,500 to £6,500 or more, depending on the span, the load above and access. This covers engineer's calculations, propping, the beam, making good and building control. Wider openings and double beams cost more.

Do I need permission to knock two rooms into one?

You do not normally need planning permission for an internal knock-through, but you do need Building Regulations approval, because the wall is almost always load-bearing and must be replaced with an engineered steel beam. A party wall agreement is also often required where the wall is shared with a neighbour.

Is the wall between two reception rooms load-bearing?

In most London terraces, the wall between the two reception rooms is load-bearing, carrying the floor and structure above. You should never assume, though. A structural engineer or experienced builder must confirm its role and specify the correct beam before any removal.

Should I fully knock through or keep an opening?

Both work. A full knock-through creates a single open-plan room, while keeping a partial wall, a wide cased opening, or twin openings either side of the chimney breast retains some definition and period character. The right choice depends on how you want to use and feel the space.

How long does a knock-through take?

The structural work itself usually takes 3 to 5 days on site, including propping, removing the wall, seating the beam and making good. Plastering, flooring and decoration follow. The calculations and any party wall notices should be arranged in advance.

Knocking Through Two Rooms