April 29, 2019

The Anatomy of a Glass Box Extension

Written by Rebecca Clayton

Posterns Court Glass Extension by IQ Glass

The IQ Glass Guide to a Glass Box Extension
anatomy of a glass box extension


Glass box extensions are very popular extension types for houses and buildings all over the UK.

A glass extension could provide a modern extension to a contemporary house or could be used to create an all-glass extension to a more traditional building; this extension method is sometimes preferred by planning authorities for the visual differential you achieve between the extension and the traditional house.

When you are looking to add a glass box to a building there are various elements that you need to think about. Take a look below for a closer look at the anatomy of a glass box extension:

The Fixing to the Building

The structural glass box needs to be connected to the existing building. Our design department designs this connection keeping in mind the glass specification and the building structure we will be connecting too.

glass connection to building

Wherever possible, the building connections are designed to be hidden by the building finishes. This creates a clear and minimal connection between the extension and the existing building.

If you are connecting a glass extension to a listed or protected building this connection is even more important to get right. There are various methods we can use to connect a glass extension to a listed building whilst protecting it. One solution could be the creation of a sub steel frame that supports the weight of the glass extension and provides an interface to the listed building.

Every connection detail is designed bespoke for your project by our in-house design team.

Glass Box Extension Hawthorns Glass Box Extension by IQ Glass

The Roof Glass

You don’t have to have a full glass roof to your glazed extension, but most ‘traditional’ glass box designs do have a fully glazed structure. This includes a glass roof.

The glass roof can be designed to span the full size of your glass extension. It can be created using a single unit of glass or you can use multiple units that are connected using structural silicone. If the connection between two panes of roof glazing is longer than roughly 1.3m you then need to integrate glass beams or steel sections into the glass box structure to support the roof.

Eton Villas is a good example of a smaller glass roof which didn’t need internal supports. You can see that the insulated glass roof units were just silicone jointed together. Due to the shorter span, no additional support was needed.

Glass Extension Eton Villas by IQ Glass

Posterns Court had a larger span, with a glass joint over 1.3m. That meant that additional internal support was needed below the joint. On this project, they opted for a glass beam within the glass extension to support the roof.

Glass beam glass box extension Posterns Court Glass Extension by IQ Glass

When you have internal supports to the glass roof the connection of the support to the building and other steel elements is very important.

A closer view of the glass beam at Posterns Court shows one example of how a glass beam can be designed to connect to a steel beam.

Glass Beam Connection Posterns Court Glass Extension by IQ Glass

This view of Drax Avenue showcases how a glass beam can interact with a plasterboard wall. The glass beam is connected to the building with a steel ‘shoe’ which has then been covered by the building finishes. This then creates a completely frameless connection.

glass box extension Drax Avenue by IQ Glass

We always recommend that some type of solar control coating be specified for any roof glazing. This type of coating reduces the amount of solar radiation that comes through the glass which stops that overheating most commonly associated with old fashioned glass conservatories.

We design all our glass roofs for maintenance loads. So, if the kid’s football gets kicked up onto the roof it is possible to access the roof to get it back!

structural glass roof

Glass to Glass Connections

A glass box extension is likely to have many glass to glass connections. The most notable of these is the connection between the frameless glass walls and the roof, and the corner joint of two elevations of the glass box.

The connection of a fixed pane of glass to another fixed pane of glass can always be frameless.

When connecting glass at a corner one pane of the insulated glass unit is ‘stepped’ over the returning unit. This creates a flush and frameless corner connection with minimal sightlines from both inside and out.

structural glass box garden room

When roof glazing is connected to a fixed vertical pane of glass (making up a wall of the glass extension) a similar 'stepping' technique is used. For roof glazing, we elongate the stepped edge of the glass unit which creates a drip edge to the glass roof.

structural glass roof

glass box extension

The Opening Doors

If the glass extension is going to provide direct access to the garden or patio then doors will need to be integrated into the structural glass construction.

You can include whatever door system you want to into a glass box extension. If you want to maintain a frameless design, then a sliding door system will give you the slimmest frames. Our minimal windows® sliding door system tends to be the most popular as it has sightlines of 21mm.


Other popular options include Sieger® bi-folding doors, Vitra Pivots or Sieger® Aluminium Casement doors. An industrial steel design can be created by using a Mondrian® Steel Door, this works well when paired with a steel supported roof.

Sieger Bifolding Doors

The Steel Support (For Opening Doors)

Whenever a door system (or opening window) is integrated into a structural glass box you also need to include a steel support around the opening. This creates the opening within the structural glass for the opening door to sit in.

In order to create a neat finish to the glass extension, these steel sections are then covered. The most popular options for cladding a steel section support are either aluminium pressings or back painted glass. Other metal materials can be used to clad the steel both inside and outside if the design warrants it. Alternative options include Architectural Bronze, Stainless Steel or Corten Steel.

structurally glazed box extension The steel support on Drax Avenue was clad with back painted glass

slim framed sliding doors The House in the Woods projects used insulated aluminium pressings to clad the steel section

The size and depth of the supporting steel required will depend on the size of the door opening and the weight of the glass roof above it. These dimensions can easily be calculated by our design team.

Connection at the Base

The glass extension will need to be connected to the ground/building structure at the base. These fixing details are designed by our design team to ensure they are as minimal as possible.

Where opening doors are included within the extension then adequate drainage is also designed into the system.

All building connection details are designed bespoke by our design team. This allows us to ensure that the connection is frameless and works for your building structure.


More Information?

If you would like more information about creating a glass box extension for your project just speak to our technical team at IQ. They are happy to provide advice on glass specifications, costing and design options.

Why not arrange a visit to the IQ Glass Showroom in Amersham? We have a number of glass extension techniques available to view, as well as our full range of opening door systems.

Get in touch with the team to book your appointment.


Useful Links: Beginners Guide to Glass Box Extensions