Glass box extensions

A glass box extension uses structural glazing with slim or concealed frames to create a room that is almost entirely glass. It is often used on period houses, where a contemporary glass addition can sit lightly against the old building and is sometimes easier to justify in planning than a solid extension.

Glass extensions are specialist work. The glass is heavy, made to measure and needs careful structural fixing, and the heat loss and solar gain must meet the building regulations, usually by calculating the performance of the whole house.

What is included

• Design with specialist glazing input
• Whole-house energy calculations where needed
• Steel or glass beams and slim-frame glazing
• Solar-control and low-energy glass specification
• Drainage details for frameless roofs
• Planning drawings that show the glass detail

Who it suits

• Period and listed-style houses where a light touch is wanted
• Small infill spaces and side returns
• Garden-facing dining rooms

Design advice

Solar gain is the risk. We specify solar-control glass, plan for opening vents or sliding panels, and avoid fully glazed roofs on south-facing extensions unless shading is included.

How much does it cost?

National guide prices including VAT at 20%. Build cost only; design, structural and council fees are extra. Your area page shows prices adjusted for your region.

Planning rules across the UK

England

In England a glazed or orangery-style extension follows the same size limits as any single-storey rear extension: 3 m or 4 m without an application, or up to 6 m and 8 m through prior approval. Building regulations limit glazing to about a quarter of the new floor area plus the openings it replaces, unless whole-house calculations show the design still performs.

Wales

In Wales a glazed or orangery-style extension follows the normal single-storey limits (up to 4 m at the rear). Welsh building regulations also limit glazing and heat loss, so glass-heavy designs need calculations.

Scotland

In Scotland, householder permitted development rules (Class 1A for single-storey ground-floor extensions and Class 1B for extensions of more than one storey) allow some smaller extensions without a planning application, within limits on depth, height and distance from the boundary. Conservation areas and listed buildings are more restricted. Separately, almost every extension needs a building warrant from the council before work starts.

Northern Ireland

In Northern Ireland the Planning (General Permitted Development) Order 2015 lets some single-storey rear extensions go ahead without planning permission, up to 3 m on a semi-detached or terraced house and 4 m on a detached house, within height and boundary limits. Larger, side and two-storey extensions normally need planning permission from the council, and building control approval is separate.

Building regulations and neighbours

Building regulations approval is separate from planning. In England you can use the council's building control team or a registered building control approver; a full plans application gets the drawings checked before work starts, and the inspector signs the work off with a completion certificate.

In England and Wales, if the work involves a shared wall, building on the boundary, or digging foundations within 3 m (sometimes 6 m) of a neighbour's building, the Party Wall etc. Act 1996 applies and notice must be served before work starts: two months for work to a party wall, one month for new walls on the boundary and for excavations. Scotland and Northern Ireland have no Party Wall Act.

Questions about glass box extensions