If you're considering extending your home, glass box extensions are a contemporary option that make a striking design statement, flood your home with light and will increase its value.
From how much you can expect to pay for a glass box extension to planning permission, design ideas and finding tradesmen to do the job, our guide to glass extensions is here to talk you through the possibilities.
For more practical advice, check our guide to extending a house.
- Find more inspiration for designing a statement extension in our gallery
How much will a glass extension cost?
While we'd recommend budgeting for a minimum of £3,000 per m², you can find a more accurate estimation for your glass extension with the help of our extension cost calculator.
'While a conservatory can be a relatively inexpensive alternative to a more solid extension, a glass box extension will almost certainly be more costly,' says Alan Cronshaw from Acronym Architecture & Design.
'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.'
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,' says Melanie Clear from Clear Architects.
'As long as it’s a considered extension, and not at the front of the property, getting planning permission for your glass extension will be no different to getting planning for a regular extension.'
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,' says Alan Cronshaw.
'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.'
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' says Melanie Clear.
'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.'
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,' says Hugo Tugman of Tugman Architecture & Design.
'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.'
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,' says Real Home property expert Michael Holmes.
'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.'
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,' say Alan Cronshaw.
'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.'
For inspiration, go to our glass extension design ideas gallery.
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,' says Hugo Tugman.
'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.'