Corner windows on a rear extension

A corner window on a rear extension removes the one element the structure wants most, which is a column where two walls meet. Everything about the detail follows from that: the goal post frame that replaces it, the deflection limit the glazing manufacturer will insist on, the thermal bridge at the junction, and the oblique view into a neighbour's garden that a corner opening creates and a flat elevation does not.

The design is achievable on almost any domestic extension. What varies is how much of the cost sits in steel and how much sits in the argument with the neighbour.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

What a corner opening does to the structure

In a conventional extension, the corner carries load from two directions and provides the lateral restraint that stops the two walls moving independently. Take it out and both jobs move to a frame.

The usual arrangement is a pair of beams meeting at the corner, supported by columns set back from the opening, with the corner itself unsupported. Load from the roof and from any wall above arrives at the corner and travels along both beams to those columns. On a single storey rear extension with a flat roof the loads are modest. Add a masonry gable above, a green roof, or a first floor, and the beam sizes grow quickly.

Lateral stability is the part that gets forgotten. A rear extension with glazing on two adjoining elevations has very little to resist racking in either direction, and the designer has to find it somewhere: a moment-resisting frame, a stiff return wall at the far end, or a diaphragm action in the roof deck. None of these is difficult, and all of them have to be designed rather than assumed.

Get the structural engineer involved before the planning drawings are finalised. The beam depth determines the ceiling height at the opening, and the ceiling height determines whether the eaves line you drew still works.

Goal post frames and hidden columns

The frame most often specified is a goal post: two columns and a head beam, fabricated as a unit or bolted on site, sized to carry the load and stiff enough to satisfy the glazing tolerance.

Where the corner is fully open, the arrangement becomes two goal posts sharing a corner, or a single frame with a cantilevered corner. The cantilever is the detail clients want, because nothing interrupts the view, and it is the detail that drives the steel weight.

Hiding the columns is a matter of planning the junctions early. A column within the thickness of a 300mm cavity wall is straightforward. A column at the junction with the existing house can usually be concealed in the reveal. A column sitting in the middle of a glazed run cannot be hidden and will be visible for the life of the building, which is an argument for resolving the layout at concept stage rather than in the structural package.

Fire protection changes the dimensions. Exposed steel in a single storey domestic extension often needs no protection, but where the frame supports a floor above or forms part of a compartment wall, intumescent coating or boarding adds thickness that has to be allowed for in the opening dimensions.

Deflection limits and glazing tolerances

This is the number that controls the design, and it comes from the glazing supplier rather than from the structural code.

Large format glazing units are intolerant of movement in the supporting structure. Suppliers typically specify a maximum deflection across the head of the opening in the order of a few millimetres, often expressed both as a span ratio and as an absolute limit, and the absolute limit is usually what governs on a wide opening.

Three consequences for the design:

• The beam is sized for stiffness rather than strength, so it is deeper than a strength calculation alone would suggest.
• Pre-cambering the beam can recover some depth, but it has to be coordinated with the glazing installation sequence.
• Dead load deflection that occurs before the glass is installed does not count against the limit, which is why the construction sequence has to be specified rather than left to the builder.

Obtain the supplier's deflection requirement in writing before the steel is designed. Designing to a generic span over 360 limit and discovering the supplier wants 3mm absolute is a late and expensive change.

Overlooking at an oblique angle

A corner opening creates sightlines that a flat rear elevation does not. Standing inside, you can see along the boundary in a direction that was previously blocked by the return wall, and a neighbour will notice.

The objection that follows is usually about oblique overlooking of their garden rather than window to window views. That argument carries less weight than a direct sightline, but it is not worthless, particularly where the garden is narrow and the part closest to the house is the part now visible.

Answer it with a drawing rather than an assertion. A plan showing the view cone from inside the corner, the boundary treatment and its height, and the distance to the neighbour's rear elevation and to their nearest usable garden space, settles most of these before they escalate.

Where the geometry is tight, the practical mitigations are a solid or obscure return panel of one to one and a half metres at the boundary end of the glazed run, a fixed fin, or simply setting the corner opening away from the boundary side and keeping the flank elevation solid. The last costs nothing and is the one clients resist most.

Set it out in the Design and Access Statement as a design decision taken in response to the boundary condition. An officer reading that the corner was positioned away from the boundary deliberately treats the scheme differently from one where it looks accidental.

Thermal bridging at the corner

A structural corner with no column is also a thermal corner with a steel frame running through it, and steel conducts.

The detail has to carry the insulation line continuously past the frame, which usually means insulating outside the steel rather than between or inside it. Where the column sits in a cavity, the cavity closer and the insulation continuity around it need drawing at 1:5. Where the beam sits above the glazing head, the gap between the underside of the beam and the top of the frame is where condensation appears if the insulation stops short.

Thermal bridging at junctions accounts for a growing share of total heat loss as elemental performance improves, and a corner frame is the worst junction in a typical extension. Either use an accredited construction detail or have the psi value calculated, because assuming a default value for a non-standard junction understates the loss.

Condensation risk at the frame is the practical failure. Clients notice water running down the inside of the glass on a cold morning, and the cause is almost always a cold bridge at the head or the jamb rather than the glazing unit itself.

Where the detail is worth the cost

Corner glazing costs more than an equivalent area of flat glazing by a meaningful margin, taking the steel, the glass and the detailing together. It is worth it in three situations.

Where the corner looks at something: a view down a garden, a tree, a change in level. Where the room is deep and the corner is what brings daylight into the back of it, which a single rear elevation cannot do. And where the house has a dark, narrow plan and the corner opens a diagonal that makes the space read as larger than it is.

It is not worth it where the view is a fence at four metres, where the orientation means the corner faces north and the gain is only in daylight at the expense of heat loss, or where the budget means the steel is paid for by dropping the insulation specification elsewhere.

That last trade happens more often than it should. A corner window in a poorly insulated extension is a worse building than a flat window in a well insulated one.

Showing the structural intent on the application drawings

Planning does not approve the structure, but the drawings have to be credible, and an officer who cannot see how the corner stands up will ask.

What to show:

1. Beam and column positions indicated on the plan, even if sizes are provisional.
2. Ceiling heights at the opening, accounting for the structural depth.
3. A section through the corner showing the head detail, the insulation line and the glazing.
4. The external appearance of any visible structural element, including its finish.
5. The view cone from the corner opening on the site plan, where a boundary is close.

Where the extension is to a listed building or in a conservation area, the corner opening will be scrutinised as a contemporary intervention, and the Heritage Statement should address it as such rather than treating it as a window. For the measurement rules that govern the extension itself, the related notes on rear extension depth from the original rear wall and eaves height limits near a boundary cover the limits a corner design has to sit within.

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