Glass box extension

What holds a glass box up

There are two ways to build one, and the difference shows in both the look and the bill.

A frameless box uses the glass itself as structure. Laminated glass beams span from the house wall to laminated glass fins or to the glass walls, the roof panels sit on top, and everything is bonded with structural silicone. Nothing metal is visible except a slim channel at the floor and the house wall.

A steel-assisted box hides a minimal steel frame in the roof edge and corners, often 60 to 100 mm box sections, and hangs the glass from that. It costs less, copes with bigger spans and sliding doors, and from most angles looks nearly as light.

Either way there is usually ordinary structural steel where the box meets the house. Opening up the back wall needs a beam or a full frame to carry the masonry above, and that part is engineered like any other extension. Our page on steel beams and RSJ cost covers it.

What a glass box costs, and why

Our guide range is £2,750 to £4,500 per m² of floor including VAT, which puts a 12 m² box at £33,000 to £54,000. That is well above the £2,200 to £3,000 per m² we use for a steel-framed room with a solid roof. Small boxes sit towards the top of the range per square metre, because the structural opening, the design work and the delivery cost much the same whatever the size.

The money goes on three things. The glass is made to order in large toughened and laminated panels, some weighing several hundred kilos. Each needs lifting gear and a specialist crew to place. And the engineering is bespoke: every panel, beam and fixing is calculated for your box alone.

Options that add to the price include solar-control coatings, triple glazing, heated glass, opening roof vents and doors without a threshold. Of these, solar control is the one not to skip on a south or west wall.

Planning permission

Glass gets no special treatment on size. Under English permitted development a single-storey addition at the back may project 3 m (semi or terrace) or 4 m (detached house), up to 4 m high, and double those depths under the prior approval route.

The catch for glass is a condition that the outside materials must be of similar appearance to the house. Conservatories are let off that condition, and whether your frameless box counts as one is the council’s call. Two sensible routes: apply for a lawful development certificate before you order, which gives you the answer in writing, or make a householder application and remove the doubt.

On a listed building you need listed building consent whatever the size. A glass box touches the old fabric lightly and is plainly new, which is one reason they turn up on listed houses so often, but consent is never automatic.

North of the border the permitted development classes differ, and no work may begin until the council has issued a building warrant.

Building regulations: heat is the hard part

Under Approved Document L, glazing in a new extension is capped at 25% of its floor area, with an allowance added back for any existing windows and doors the extension blocks. A glass box goes many times past that.

It can still pass. The designer shows by calculation that the house with its new box will lose no more heat than the same house with a compliant extension. In practice that means high-performance glass, and often improvements elsewhere in the house, such as more loft insulation or better windows upstairs, to make the sums balance.

The other route is the conservatory exemption. A glazed addition at ground level under 30 m², shut off from the house by external-quality doors and not served by the house heating, is outside the energy rules. Most people buying a glass box want it open to the kitchen, so the exemption rarely fits.

Structure, safety glazing and electrics go through building control in the normal way. Roof glass is laminated so that a broken pane stays where it is.

Living with one

Heat in summer is the complaint to design against. Ask what solar-control glass is proposed and how much of the sun’s heat it lets through, and plan for an opening vent or doors on two sides so warm air has a way out. North-facing boxes are far easier.

In winter a well-specified box is comfortable to sit in, though it will never hold heat like an insulated wall. Underfloor heating suits it, since there is nowhere to hang a radiator.

Glass roofs are laid to a slight fall so rain clears itself and carries dust with it. You will still be cleaning the roof a few times a year, and you should think about what overlooks it.

The alternative worth pricing

Ask for a second price on a steel-framed room with an insulated flat roof, one large rooflight and sliding doors across the full width. It gives most of the daylight, passes the energy rules without a struggle, and usually costs a third less. Many people who set out wanting a glass box end up building this instead. The cost calculator prices both.