Alternative Use of Glass in Extensions

The use of glass can be a key aspect to any extension, allowing more light into a longer plan, existing spaces in the house and the new extension. This is usually done in two forms; the use of a rooflight in a flat or pitched roof extension allowing light further into the building or large glass doors such as sliding or bifold doors on the rear of an extension opening and looking out onto a patio or garden.

However, there are many other inventive ways to use glass within an extension to maximise daylight into a new extension and create more space within an existing house. Some of these options are explored below and within our new detail set. 

01 Hidden Gutters on Chimney House

There are many ways to use glass within an extension to maximise daylight…

Glass Window Box

A glass box window seat can be an excellent way of creating an additional space within an existing building or as part of an extension. As the glazing will be oriented in three or even four directions, it is also a great way of increasing luminosity into a space. Many are finished at seating height, roughly 450mm above the finished floor level, with a timber sill and decorated with cushions to create a cosy reading or seating area. 

Box Window Section Detail

If this is being built on an existing wall, no new foundations will be needed which can help cut costs whilst adding some extra space and light. However, an engineer will have to be consulted as the base cantilever will need a structural solution based on the wall or existing foundations type and condition as well as the size and weight of the glass box. This is not usually something the glass supplier / manufacturer will do although they will be able to give you the weight of the glass box and approve the required connections to the existing and new structure. 

Lean to extension / Glass box extension

These glass additions can be great if your house already has a historic extension. This is especially in the case of terraces or semi-detached houses with only a small amount of space between the existing L-shape rear and neighbouring properties. They can come in many shapes, configurations and sizes to suit any project and existing house.

A large completely clear, frameless extension space with no steel supports can be created using pieces of structural glass with laminated glass beams and fins; creating a sense of light and space. 

3 Glass Box Extension
Lean to Extension detail

Unlike a traditional glass conservatory, modern glass box extensions use double and triple glazing, maintaining a high level of thermal performance all year round. Different coatings and glass specifications can also be used to increase performance and reduce maintenance.

Solar control coatings can provide effective solar protection, low maintenance coatings help lower maintenance and cleaning on the glass and heated glass can remove the need for the addition of heating to the additional space in winter.

Due to the amount of structural glass needed, these can be an expensive option for a house extension, although the light and space they create can be beautiful.

Pivot Door

A glazed pivot door can help blur the boundaries between internal and external spaces. Unlike sliding doors where one or maybe more glass panes need to remain closed or fixed, pivot doors can be fully open, allowing for maximum air and connection to external spaces. Using a system with a minimal frame can help increase the open area of glazing and give an almost frameless feel to the glass. 

The pivot mechanism is also a very strong element and therefore allows installation  of extremely large glass sizes. With all glass additions and especially glass pivot doors, it is always key to remember that the pieces of glass can weigh a lot and may need special equipment for installation.

The structural glass supplier / manufacturer should be able to estimate glass weights, helping to inform how the glass will reach the rear; over, through or around the house and any machinery needed for this such as a crane. 

The Building Regulations and Overheating

Approved Document Part O specifies requirements relating to overheating.

Requirement O1 states:


Reasonable provision must be made in respect of a dwelling, institution or any other building containing one or more rooms for residential purposes to –

a) limit unwanted solar gains in summer

b) provide an adequate means to remove heat from the indoor environment.


In meeting the obligations in paragraph 1

a) account must be taken of the safety of any occupant and their reasonable enjoyment of the residence

b) mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it.

There are two methods to demonstrate compliance with requirement O1.

a) The simplified method for limiting solar gains and providing a means of removing excess heat through area weighted heat loss calculations.

For full notes refer to Approved Document Part O.


b) Dynamic thermal modelling, using whole house thermal calculations prepared by a certified SAP assessor.


Acceptable Strategies for Reducing Overheating Risk

Approved Document Part O lists a selection of acceptable strategies for limiting solar gains:

  • Fixed shading devices; shutters, external blinds, overhangs, awnings.
  • Glazing design; size, orientation, g-value, depth of window reveal.
  • Building design – placement of balconies for example.
  • Shading provided by adjacent permanent buildings, structures or landscaping.


Excess heat should be removed by the following means:

  • Opening windows
  • Ventilation louvres in external walls
  • Mechanical ventilation system
  • Mechanical cooling system

Detailing Glass in Extensions

Glass can be a great if not essential material in any extension. There are many ways of utilising it that go beyond a simple rooflight and large patio doors. We have created a set covering the details above; glass box extensions, glass lean-to extensions and minimal frame pivot doors to help in the detailing and construction of these elements.

For More Inspiration…

For More Inspiration…


Check out our Pinterest board with amazing real built projects to help inspire your next glass extension.

Other Resources

Below is a list of suggested manufacturers that offer structural glazing for glass box extensions and/or pivot doors. 

IQ glass 

Glazing vision 



Glass extensions Uk 

Uk Glass Rooms 

J Price Group

Room Outside

Glass Extensions

Fine Point 

Uk Structural Glass 

Glass Uk 

Glass Space 

There are other options for glass within extension such as rooflights, walk on rooflights and clerestory glazing.

Rooflight options

Vario by Veluz []

The rooflight company []

Rooflight Architectural []


Written by Aida Rodriguez-Vega, architect and researcher. At the Detail Library, Aida keeps busy by carrying out technical research and drawing new details for the ever-growing library.

Image Credits

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Architect credit: Emrys Architects []

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Architect credit: Unknown

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