Open-Plan Conversions London

An open plan conversion in London nearly always means a knock through: taking out a wall that holds the house up and carrying its load on a steel beam you never see. This page is for the London homeowner who wants two rooms made into one, or a kitchen opened into a dining room, and who needs to know what is involved before getting quotations. It covers which walls in a London house are doing structural work, how the job runs on site, the building regulations and party wall duties, what it costs and what goes wrong. We write it as the contractor who props the floor and cuts the opening. A structural engineer we appoint designs the beam, we build to the design, building control inspects it. If the wall turns out to be a stud partition, you may not need us, and we say so.

Across London, a knock-through finished as one room typically costs £3,000 to £8,000 or more, including the steel beam (indicative ranges, MyBuilder and Checkatrade, 2026). GS Renovation Ltd, a building contractor based in Potters Bar, does the work in London: the structural engineer we appoint designs the steel, we install it, and building control signs it off.

Which walls come out in a London knock through?

The wall you want gone is almost always doing something, and in a London house it is one of four. In a Victorian or Edwardian terrace the spine wall between the two reception rooms carries the first floor joists, and removing it is the classic front to back knock through. The rear wall between the back reception and the closet wing kitchen carries the closet wing roof and often a bedroom, and opening it makes the kitchen diner. The hall wall carries the stair and the floor above and is the one that changes the fire escape route. In an interwar semi the wall between the two reception rooms usually is load bearing, and the wall between kitchen and dining room usually is not.

Flats are different again. In a mansion block or a converted house the cross walls carry the floors of every flat above you. The engineer is designing for loads you cannot see, and the freeholder has to consent. What you get from the opening depends on how much wall you keep. A full width removal with the beam hidden gives one room. Keeping a nib either side and a boxed downstand reads as two connected rooms and costs less. A broken plan opening, with a glazed screen or a wide cased opening in the new beam, keeps sound and cooking smells apart while letting the light through. We recommend broken plan where the hall wall is involved, for the fire reasons below.

How an open plan conversion runs on site in a London home

The sequence starts with a survey and ends with a completion certificate. At the survey we trace what the wall is carrying: joist direction, the wall above, the roof, and where the load lands at each end. The structural engineer we appoint then designs the beam, its padstones and its bearings. Where two walls meet at a corner, as when a rear wall and a closet wing flank come out together, the design is usually a goalpost frame rather than a single beam. The engineer also decides between a downstand beam below the ceiling and a flush beam set within the floor depth. We build whichever is drawn, and we tell you the cost difference between the two before you choose.

On site we prop both sides of the wall and the floor above before anything is cut, then take the wall down in a controlled strip. The beam goes in on its padstones, is packed tight with dry pack mortar so the load transfers before the props come out, and building control inspects it exposed. Only then does the finishing start, and finishing is most of the programme. The floors of the two rooms are often at different levels and have to be brought to one. Radiators and pipework in the old wall are rerouted, sockets and switches moved, and the ceiling joined and skimmed. A flush beam adds joist hangers and a second inspection. Decoration comes last, after the final building control visit.

Consents and regulations for a knock through in London

You need building regulations approval for every structural opening, planning permission for none, and a party wall notice where the beam bears into a shared wall. The beam comes under Part A. Part B matters when the hall wall comes out, because a house with two or more floors above ground normally relies on a protected hall and stair as its escape route. Open the hall into the living room and that route is lost, so building control may ask for a fire door arrangement, a sprinkler, or a broken plan layout that keeps the enclosure. Part F covers extraction where a kitchen joins a living room, and Part P covers the electrical work. On a listed house, listed building consent is needed.

Where the new beam bears into the wall you share with next door, the work is notifiable under the Party Wall etc. Act 1996. You serve the notice as building owner, two months before the work starts, and the neighbour can consent or appoint a surveyor. We are not party wall surveyors and we do not serve notices, but we provide the engineer's drawings the notice relies on and we build to the award. If the wall is a timber stud partition with nothing bearing on it, none of this applies and a carpenter can take it out. The way to know is to have it checked, not to guess from how hollow it sounds. We check at the survey.

What does an open plan conversion cost in London?

A complete knock through in London costs £3,000 to £8,000+ including the steel beam and the finishing, indicative London ranges, MyBuilder and Checkatrade, 2026. That is not a quotation. A firm price follows a survey and the engineer's design, because the drawing decides half the cost. A single downstand beam on a short span, with padstones on sound brick, sits at the bottom of the range. A goalpost frame at a corner, a flush beam within the floor depth, or a long span over a full width opening moves the job up it. The finishing moves it further. Two floors at different levels, a boiler flue or radiator pipework in the old wall, and a ceiling that has to be joined all add days.

The figures that sit outside the building work are the engineer's fee, the building control charge and, where the neighbour appoints a surveyor, the party wall award. A parking suspension for the steel delivery and a skip permit are small but real on a London street. If you are refitting the kitchen at the same time, the sequence matters more than the money. The kitchen layout, the island position and the extraction route decide where the beam can land and where the services run, so the kitchen design should exist before the engineer draws. The most common way to overspend is to open the wall first and design the kitchen second, then pay to move a steel post that is in the way.

Living in your London house during the knock through

You can stay in the house, but the ground floor is a building site until the plaster is dry. If the kitchen wall is the one coming out, you are without a kitchen for the duration. The steel sets the access. A beam longer than the hall is delivered through a front bay window or spliced on site to the engineer's detail. Either way it needs a parking suspension on the day, because it comes off a lorry. Most London terraces have no side access, so every barrow of rubble and every length of steel comes through the front door. We sheet the hall, seal the stair with a zipped screen and run extraction to a window so the dust stays downstairs.

The neighbours will hear it. Cutting brickwork against a party wall carries straight into the room next door, so we tell them the programme before we start and keep to the hours the borough's noise rules allow. In a flat the managing agent will want the engineer's drawings and our insurance before we book anything, and the lift is usually off limits for rubble. Propping stays in place until building control has seen the beam, which means a few days of steel props in the room you are trying to live around, and a ceiling you cannot yet paint. Plan the kitchen fit and the decorating so that the day the props come out is not the day you expected to cook. If the new kitchen is already ordered, our kitchen fitters page covers how the fit follows the structural work.

What goes wrong with open plan conversions, and how to choose a London contractor

Knock throughs we are asked to look at after someone else built them fail in a handful of ways. A beam sized by the builder rather than an engineer, usually too shallow, so the ceiling above cracks along a line and the doors upstairs start to stick. A beam bearing on a half brick nib with no padstone, so the brickwork crushes. A floor above that was never propped during the cut, so the joists dropped before the beam was in and the crack is permanent. A hall wall removed on a three storey house with no thought for the escape route, found when the house is sold. And two floors joined with a step because levelling them was never priced.

Choosing a contractor for a knock through is a matter of asking who designs it and who signs it off. Expect a named structural engineer, not a builder who has done plenty of these. Expect a building control application number before the wall is touched, and two inspections, one with the steel exposed and one at completion. Ask whether the quotation includes rerouting the heating and electrics in the wall, levelling the floors and joining the ceiling, or whether those are extras waiting to happen. Ask who will be on site each day and who you ring. We give you one named point of contact, our own trades rather than a subcontractor chain, and building control coordinated on every notifiable job. The notice you serve where the beam bears into a shared wall is explained on our party wall page.

• Structural engineer coordination and building control sign-off
• Open-plan kitchen-diner conversions
• Rooflights and glazed roofs to bring light deep into the plan

Next steps that tend to follow open-plan conversion in London: home renovation in London, side return infills, garage conversions and Knock-Through Guide.

Why London Homeowners Choose GS Renovation

London homeowners choose GS Renovation for an open plan conversion because we treat the knock through as the structural job it is and price the finishing honestly alongside it. A structural engineer we appoint designs every beam, we build to that design with our own trades, and building control is coordinated on every notifiable job. Abdelaaziz Saaid, our founder and director, brings over 30 years of hands-on structural experience to the survey, and a free, no obligation written quotation follows it. One named point of contact runs the job from survey to certificate. Fully insured, and rated 4.9 from 31 reviews on MyBuilder.

• Our own trades on site rather than a chain of subcontractors
• Broken-plan and Crittall-style options priced alongside full open-plan
• Beam depth checked against ceiling heights before you commit

Checked by Abdelaaziz Saaid, founder and director of GS Renovation Ltd (Companies House no. 14611254). Structural design is by the consulting engineer appointed to your project.