Rear wall knock-through

Knocking through to a rear extension

Joining a rear extension to the house means removing some or all of the original back wall, which holds up the floors and roof above. A structural engineer designs a steel beam (and sometimes posts) to carry that load, the wall is propped while it is cut out, and the beam sits on padstones before being fire-protected and plastered.

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Back wall of a house removed during an extension build with a red steel beam on padstones and temporary props

How is the back wall removed safely?

Temporary props and needles support the wall above. The opening is cut, the steel is lifted in and packed tight onto padstones, and the props come out only when the packing has cured. Building control inspects the steel and its bearings before it is boxed in.

The beam size depends on the span and what sits above it: a single-storey back wall with a bedroom above needs a smaller steel than a wall carrying two floors and a roof. Long spans sometimes need a goalpost frame.

How much does a knock-through cost?

As part of an extension, allow £3,500–£9,000 including VAT for the engineer's calculations, steel, padstones, propping and making good. Wide openings and goalpost frames are at the top of that range. Building control fees are separate.

Building control

Every rear extension in England needs Building Regulations approval, whether or not planning is needed. You can use the council's building control service or a registered building control approver, by full plans (checked before work, usually within five weeks) or a building notice, and you get a completion certificate at the end.

What is included?

Structural engineer's calculations
Temporary propping
Steel beam or goalpost frame
Padstones and packing
Fire protection to the steel
Plastering and making good
Building control inspection
Dust screens and floor protection

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FAQ

Rear wall knock-through questions

Can the whole back wall come out?

Often, with a beam across the full width or a goalpost frame. Some walls need a short pier left at the ends.

Do I need building control for a beam?

Yes. The calculations and the installed steel are checked.

How long does it take?

The wall itself usually comes out in a day or two, timed for when the extension is weathertight.

Will there be cracks upstairs?

Minor hairline cracks can appear as the beam takes the load; we make good at the end of the job.

Can the beam be hidden?

Yes, within the ceiling depth (a flitch or a downstand boxed in) where the floor allows.

Does the steel need fire protection?

Yes, usually with fire-rated board or intumescent paint.

Where