How to get the perfect glass box extension

Find project inspiration and specialist advice for your own glazed addition

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Gregory Phillips Architects was tasked to design a luxurious outdoor-feel space with column free glazing for this Wimbledon home. A reflecting pool helps blur the boundaries between inside and out

How much will a glass extension cost?

While a conservatory can be a relatively inexpensive alternative to a more solid extension, a glass extension will almost certainly be more costly.

You will probably want it to flow from the rest of the house, which may mean compensating for the areas of glass by increasing insulation levels elsewhere or carrying out other energy-saving measures. High-specification glass, the structural solution and alterations to the rest of the house are all costs that are likely to be greater than if you build something more conventional.

Fees are likely to be higher for the architect or designer and structural engineer, too, as more work will be involved. You may also have to employ a thermal consultant, unless you’re building something more solid. I would advise a minimum budget of £3,000 per m².

Alan Cronshaw, Acronym Architecture & Design

glass box extension on a period farmhouse

Small windows typical of its period meant this farmhoise was quite dark, so the owners wanted an extension that would introduce daylight. Apropos proposed a glass box extension with clean, modern lines. Solar control glass keeps the space comfortable year round.

A similar project would cost £35,000

(Image: © Apropos)

Click here to get a comprehensive extension cost calculation using our free extension cost calculator


Related articles: The ultimate guide to extending your home | How to add a two storey extension | Create the perfect kitchen extension | Designing a single storey extension

A glass box extension with oak framing by Carpenter Oak

Living Space Architects created a contemporary extension for this Cornish house using a British sweet chestnut frame from Carpenter Oak teamed with floor-to-ceiling glazing.

The owners wanted to open their house to the landscape in the most sustainable way possible with an open-plan extension. The glass-box-style design includes sliding glass panels that open up the corner of the extension for uninterrupted sea views.

Budget at least £20,000 for something similar

(Image: © Living Architects)

Planning permission for glass extensions

Planners like glass extensions to bridge connections between architectural styles or to add to a home with a very dominant existing style. Conservation officers also like the visible difference between traditional architecture and modern glass.

As long as it’s a considered extension, and not at the front of the property, then getting planning permission for your glass extension will be no different to getting planning for a regular extension.

Melanie Clear, Clear Architects

a link extension to some converted brick farm buildings

To give their home a redesign, the owners of this barn conversion in Essex called in Clear Architects. Planning permission was gained for an annex building, and, due to restrictions within the green belt, the architects created a flat-roofed, single storey glass link extension to the house.

The buildings either side of the link are in brick and weatherboarding, so the glazed extension adds contemporary detail to the farm-like buildings. Wood framing ties the glazed structure to the oak details of the main property.

The oak frame, glazing and roof window cost £27,420

(Image: © Clear Architects)

What are the benefits of a glass extension

This type of glass extension will make a real design statement. It’s hard to beat the wow factor of a substantially or totally glazed extension. It can bring glamour to any project and can also be used as a device to connect two or more solid parts of a house.

If your property has a good view, floor-to-ceiling glass, or a glass roof, is a great approach to bringing these external elements into play. Using substantial areas of glass is also a way that you can build an extension to a historic building in a contrasting material that may be more subtle than something traditional.

Alan Cronshaw

Zinc clad glass box side extension

The owners of this home asked Des Ewing Residential Architects to reorganise their home and open it up to the garden. The result was this simple, but dramatic, contemporary extension, with two-storey windows and a double-height space. Aluminium-framed glazing and zinc cladding keep it maintenance free.

A similar project would cost from £150,000

(Image: © Des Ewing Residential Architects)

a stone cottage with a contemporary glazed extension

Architects van Ellen + Sheryn were asked by the owners of this Dartmoor farmhouse to design a single-storey steel- framed glass structure between the farmhouse and barn, creating a contemporary kitchen and living room with direct garden access.

Located within a National Park and Conservation Area, the modern extension was supported by planners as it is between two clearly distinct original granite buildings.

Work to the farmhouse, barn and glass box cost £780,000

(Image: © van Ellen + Sheryn)

Will a glass extension work on my house?

Consider the age of the property and the type of use the glass extension will have. For example, a south-facing glass extension will receive more sun, and so risks becoming very hot inside. A north-facing extension with underfloor heating can be a nice, light addition to a property

If your glass extension is part of a kitchen remodel, make sure the units are positioned in the centre, as you won’t be able to place them against a glass wall.

Using glass as a feature can be a great addition to a period property, and in a 1970s-style home, a framed glass extension can work well with the expansive sections of glazing in the existing building.

Melanie Clear

glass extension on a roof

When this dilapidated Victorian warehouse in east London was converted into two houses, outdoor space was required.

Clear Architects proposed a sliding glass roof box to provide access to a lofty terrace. The homes are in a Conservation Area, so the height of a roof extension was limited by planning constraints. Glass was the only suitable material, as the box needed to open for head-height, and it also makes a striking additional feature.

Glazing cost around £29,700

Be aware that you’ll be more exposed to the elements

Even when using specially formulated glass in double- or triple-glazed systems, more heat tends to be lost through glass than through a solid construction. Modern building regulations require quite high levels of thermal efficiency, and it can be difficult to get a completely glass extension to comply.

More of a problem than this can be glare and solar-gain in the summer. Even here in the UK, I have seen many such extensions where the owners have had to install blinds to offer shade.

Timber framing will give more opportunities for cosiness. Similarly, if you go for pure glass walls beneath a solid roof, or even a solid roof with some openings in it, rather than a pure glass roof, it is easier to conceal curtain tracks or blinds recessed into the ceiling that can be drawn down or across for added privacy, shade or warmth.

Hugo Tugman, Tugman Architecture & Design

striking rendered clad and glass extension to a family home

A single-storey and a double-height extension were designed for this family home by Granit Architects. A glass box was created at the point where the house changes orientation. A sliding pocket door can be drawn across the glass to make the space cosy in winter.

A similar extension would cost from £3,000/m2 for construction and materials

(Image: © Granit Architecture)

Who can build a glass extension?

Glass box extensions will usually involve specialist design detailing. So while you and your architectural designer may produce the layout and elevations for a glass extension, you’ll need to find a designer, manufacturer or contractor to assist with details such as door systems and glazed walls.

Find a glazier

The degree of specialism will depend on what you want to achieve: a structure made entirely of glass, including load-bearing structural glass beams and columns, is very specialist, and there are only a handful of companies that produce this kind of work.

A contemporary glass extension built with a more conventional structure, such as a steel frame, but with floor-to-ceiling glazing and sliding or folding-sliding glazed doors, is less complicated, and any suitably experienced residential architect and structural engineer can help, working with a glazing specialist or door supplier.

Micheal Holmes, Real Homes property expert

glass box extension to a traditional brick home

The family that owns this south-west London home asked IQ Glass for a rear frameless glass extension with high thermal performance.

The sliding access door was positioned at the side to create a more frameless appearance, and allow the use of glass beams and fins internally. The slim-framed doors also mirror the frameless look that the beams and fins have created.

The project cost around £54,000 for glass, construction and sliding doors

(Image: © IQ Glass)

Glass extension design

Although the major element of your extension is glass, there are still plenty of design options. The expanses of glass will probably sit in some kind of frame, and it is likely that you will want this to be as thin in profile as possible. Powder-coated aluminium is often chosen where the paint colour is applied in the factory, giving a wide range of shades.

The glazing can have different coatings that will affect the look of the glass extension, from transparent to reflective.

Whether the glazed panels are fixed, bi-fold, or slide open, will also have an impact on the frames and how the completed glass extension appears.

If you have a solid roof above the glazing, the structural solution for supporting this will also be a key part of the design, perhaps overhanging to shade the glass from the sun. A brise-soleil, or sun shield, can also be incorporated to perform this function, which will again change the character of the project.

Alan Cronshaw

brick and glass extension to a London home

As part of a refurbishment, this north London family asked Lipton Plant Architects to create an open-plan space leading out to the garden of their home. To obtain a level finish, the floor of the entrance room was lowered to meet that of the rear room.

A similar project would cost from £3,000/m2

(Image: © Lipton Plant Architects)

Glass as a building material

While glass can be hard and strong in certain directions, it is rigid, brittle, heavy and unforgiving to movement. This makes it more difficult to install than most other building materials.

Uninterrupted areas of glass need to be transported and installed in the form of large, heavy sheets, and often the use of a crane is necessary, which can add to costs significantly.

Hugo Tugman