Opening up a rear wall: steel beams, planning and party walls

Removing a rear wall is, in planning terms, usually the least interesting part of an extension. The planning officer looks at the box you are adding, the glazing you put in it and what the neighbours will see. The structural work behind it sits with building control, and if the beam bears into a wall you share with next door, the Party Wall etc. Act 1996 brings a separate two-month notice period that has nothing to do with the council's planning department. Keeping those three regimes apart is what stops a householder application stalling.

What planning control actually looks at

Development is defined by section 55 of the Town and Country Planning Act 1990 as building, engineering, mining or other operations, plus any material change of use. Section 55(2)(a) takes out works that affect only the interior of a building or do not materially affect its external appearance. So taking out an internal spine wall and propping the first floor on a steel beam is not development at all. No application, no fee, no eight-week wait.

A rear wall is different because it is an external wall. Replace it with a run of sliding doors and you have changed the external appearance, which is development and needs either planning permission or permitted development rights under the Town and Country Planning (General Permitted Development) (England) Order 2015. Schedule 2 Part 1 Class A covers the enlargement, improvement or other alteration of a dwellinghouse, and a new opening in a rear wall normally falls inside it.

Two things remove that comfort. An Article 4 direction can switch off Class A for houses in a defined area, which many councils have done across conservation areas. And if the house is listed, section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 catches works affecting its character as a building of special interest, inside as well as out. For a listed house, removing an internal wall does need consent, and the application will want a Heritage Statement explaining what fabric is lost.

Structural calculations sit with building control

Nobody at the planning department will read your engineer's calculations. The approval that matters for the beam comes through the Building Regulations 2010, either as a building notice or a full plans submission to the council's building control team or a registered building control approver. Approved Document A covers structure.

What the engineer produces for a typical rear opening is short: a load take-down, a beam size, a bearing length, padstone dimensions and a deflection check. Deflection is usually the governing case rather than bending strength, because plaster and brittle finishes crack long before steel yields, so engineers commonly limit the sag to around span over 360 under imposed load. A universal beam is the default. Where the floor zone will not take the depth, or where there is no way to get a four-metre length of steel through the house, a flitch beam of timber plates bolted either side of a steel flitch plate does the same job in a shallower section and arrives in pieces.

The beam also needs fire protection. Approved Document B expects the beam to carry the same fire resistance as the floor it supports, which in a two-storey house is 30 minutes and is normally achieved with two layers of plasterboard rather than anything exotic. Encasement details matter at the ends, where the steel meets masonry.

Keep the completion certificate. It is the document a buyer's solicitor asks for years later, and the absence of one is a far more common problem at sale than any planning point.

Beams bearing into a party wall

This is where householders get caught. If the beam bears into the wall you share with the house next door, you are cutting into a party structure, which is notifiable work under section 2(2)(f) of the Party Wall etc. Act 1996. You must serve notice on the adjoining owner, and the notice period is two months before the work starts.

Excavating for the new extension foundations is separate. Section 6 covers excavation within 3 metres of the neighbouring structure where you will go deeper than their foundations, and within 6 metres where a 45-degree line from the base of their foundation would be cut. Those notices run for one month.

If the neighbour does not consent in writing within 14 days they are deemed to have dissented, and a party wall surveyor has to be appointed to produce an award. That process costs money and takes weeks, and planning permission does not authorise any of it. Permission is a public-law consent to develop. It gives you no private right to touch someone else's wall, and the two timetables need to run in parallel rather than one after the other.

Lintels and openings in a flank wall

A new window in a side wall is a planning question before it is a structural one. Condition A.3(b) of Class A requires that any upper-floor window in a wall forming a side elevation of the house is obscure-glazed and non-opening below 1.7 metres above the floor of that room. Officers check this against the drawings, and a clear-glazed side window at first floor is one of the easier reasons to lose permitted development status.

Ground-floor flank openings are not caught by that condition, but they attract objections about overlooking into a side passage or a neighbour's garden, which is the same argument that comes up over raised decks and terraces. The approach to overlooking and Juliet balconies applies here too: screen it, set it back, or draw the sight lines and show they do not reach anything private.

Structurally, a flank opening in a cavity wall takes a combined steel lintel carrying both leaves, with end bearings of at least 150 millimetres and a cavity tray over. Where the opening sits under a point load from a roof purlin or a beam above, the lintel manufacturer's load table will not cover it and the engineer has to size a section instead.

Cold bridging where the old wall was

The line where the old rear wall stood becomes a thermal weak point. The ground bearing, the beam and the new ground slab all meet there, and if the insulation does not run through, you get a cold strip at skirting level and condensation on the inside face in January.

Approved Document L sets the fabric standards and expects continuity of insulation at junctions. There is a second trap for anyone keeping part of the old wall: a retained wall that you renovate over more than half of its surface counts as a renovated thermal element and has to be brought up to the current standard, not left as it was. That can mean internal insulation and a loss of 100 millimetres of room width you had not budgeted for.

Where the detailing matters most is around the steel. A beam that passes through the insulation line conducts heat straight out, so the encasement needs to be insulated as well as fire-rated. This sits alongside the drainage and rainwater decisions that come with a new roof, where the build-up depth interacts with the beam position too.

When the structure drives the elevation

Officers assess the drawings in front of them. If the engineer later specifies a beam 150 millimetres deeper than the architect assumed, the head height of the new doors drops, the glazing proportions change, and what gets built no longer matches what was approved. That is a condition breach, fixed either by a non-material amendment under section 96A of the 1990 Act or by a section 73 application if the change is more than trivial.

The fix is simple: get an indicative beam size from the engineer before the elevations are finalised. Corner openings are the usual culprit, because they need a goalpost frame or a portal with moment connections rather than a single beam, and the frame is deeper. On a flat-roofed extension sitting within 2 metres of a boundary, a deeper frame can push the roof build-up past the 3-metre eaves limit in Class A and turn permitted development into a full application. The same tension between structure and elevation makes front extensions harder than rear ones, where the frontage is visible from the street.

The documents a householder application needs

An ordinary householder application is short. You need the forms, the ownership certificate, a location plan at 1:1250 with the site edged red, a block plan, existing and proposed plans and elevations at 1:50 or 1:100, and the fee. Structural calculations are not on the national validation list and should not be submitted.

A Design and Access Statement is not required for a straightforward householder application. Article 9 of the Town and Country Planning (Development Management Procedure) (England) Order 2015 requires one where the development is in a conservation area or World Heritage Site and involves one or more dwellings, and for every listed building consent application. A Planning Statement is optional at this scale but earns its place where there is a policy argument to make, such as a design guide that the scheme departs from for a reason.

Illustrative example: a 1930s semi with a 3.5-metre single-storey rear extension, the whole rear wall removed onto a steel beam bearing into the party wall at one end. The planning side is a householder application with plans and elevations. The building control side is a full plans submission with beam calculations and a Part L junction detail. The party wall side is a section 2 notice two months ahead plus a section 6 notice for the foundations. Three timetables, one job.

Enter your postcode on PlanScope to see which reports a site like yours is likely to need before you submit.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate under section 192 of the 1990 Act is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.