Glass box extensions - how to create the perfect addition for your home

Glass box extensions come in many shapes and sizes, but they all need to start with this expert advice for adding light and space to your home

glass box extensions to create an open plan kitchen with wood rafters
(Image credit: David Money Architects)

Glass box extensions are a stunning addition, bringing added space and light into your home. You may want to create extra living space, an open-plan kitchen diner, or maybe you want better connectivity to the outside. Glass extensions can also connect rooms in a contemporary way, giving you usable spaces you didn’t have before. 

There are four main types to consider when thinking about extension ideas - single rear, side return, double storey and wraparound. But there are also glazed balconies, atriums, contemporary oriel or box windows, a glazed rooftop addition – anything is possible.

The great thing about extending is the opportunity to create a brand new feel to your home.  Glass box extensions can create a better flow with clever zoning areas to suit your family’s needs. 

Glass box extensions - the step by step guide to creating one

Kitchen in glass box extension

(Image credit: Graham Atkins-Hughes)

Although they are seen as contemporary architectural ideas for homes, these clear extensions suit traditional builds too. Due to the inherent appearance of glass – it’s clear – it allows you to see the traditional build behind it so there is a marked difference between what is new and what is old. These types of frameless glass extensions are popular with planning officers that don’t wish to disrupt the integrity of the building’s original architecture.

1. Consult an architect

Open plan box extension leading onto the garden

(Image credit: Yoko Kloeden Design)

In the initial stages, consult an architect who has worked on glass additions before as they have the experience of what can be achieved, knowledge of local planning and building regulations and may be able to recommend a specialist glazing company to create your dream addition. But also, do your own research to make sure you have as much information as possible before you get started, to prevent expensive mistakes later on.

 ‘Building onto your home might make all the difference in getting what you want without the cost and upheaval of moving,’ says Keith Myers of The Myers Touch. ‘A new kitchen addition, for example will make you feel like you are in a brand new space.’

2. Make light the focus of the design

glass box extension with glass door and roof and plants outside

Design by Granit Architecture + Interiors

(Image credit: Andrew Beasley)

Natural light has been proven to have both mental and physical benefits which has been increasingly important during the pandemic, where the home became our living space, home office and exercise area. 

‘Incorporating glass box extensions, links or roofs to a project gives that space a bond with nature,’ says Shannon Normoyle of IQ Glass. ‘The links with biophilic design - a growing trend - focuses on mimicking nature indoors, and using glazing is a brilliant way to achieve this.’

3. Start your calculations

glass box extensions with a white kitchen

(Image credit: Design Space London)

Depending on the design glass box extensions, its glazed walls, roofs and door systems will need to be thought about. Glass is a wonderful, eco-friendly and sustainable material, but it can be harder to work with than other building materials. It’s heavy and comes in large pieces, so it will need specialist transportation and often need to be craned in, all of which will add to your costs, even with small additions.

Calculations should be carried out for glass efficiency, insulation and how much load it will bear in the early stages. ‘Any additional electrics should be certified and this can often mean upgrading your current consumer units in accord with current regulations,’ says David Conlon, founder of enmasse bespoke. ‘What this all means is that things are done properly. And when you come to look at the true value of your new investment or have your home valued to sell for example, you'll have all the relevant documentation to prove this is a legitimate addition to your home.’ 

You can buy ‘off-the-shelf’ glass box extensions. It’s a more cost-effective way to build an extension and will save time and money. They start from around $1100 per square foot.

4. Add a window seat

House in Norway by Oliver Heath Design

(Image credit: Oliver Heath Design)

Glass box extensions aren't just cavernous spaces. They can have nooks and nuances, zones, and resting areas. Even a small glazed area can make a huge difference in a smaller property. 

Focussing on window seat ideas creates a cosy, informal seating area. ‘Creating a bespoke, oak-lined, oriel window adds a tactile element to a minimal space,' says Mark Smyth, co-founder of Studio Bua.

Remember also to keep some budget for tidying up and landscaping the extension looks onto. If you're having a glass box, you'll need to sharpen your garden ideas, too. And think about external lighting.

5. Add character to glass roofs

glass box extensions with a wood kitchen and wood rafters on a glass roof

(Image credit: David Money Architects)

Glass roofs can be fully glazed or partially glazed depending on the style you want and the restrictions and planning restraints of your property. Even ‘small’ changes such as roof lights, lanterns and skylights can make a huge difference to the amount of light that floods in.

This design, above, is more soulful take on kitchen ideas than simply a big glass box. is a modern take on roof glazing rafters. ‘The wood used – oak - is the same throughout, including the kitchen units, for a wonderful cohesive look,’ says the architect David Money. ‘The oak faced plywood glazing rafters support each gasket of the roof glazing system above and also conceal them so you only see the sky through the glass.’

6. Put your glass box on top of the home

glass box extensions on top of a house

(Image credit: Granit Architecture + Interiors)

A glass roof extension can totally transform the upper part of a house, and it’s a great idea for creating space and light in a roof top apartment. It will need planning however as it’s a more complicated construction. Using structural glass beams will give an uninterrupted view and make the space seem larger than it already is.

This extension creates a wonderful light-filled space with the added benefit of a roof terrace. A set of connected contemporary glass dormers extended the existing room at the back of this house to create a rooftop living room and study and an en-suite bedroom, replacing an existing loft bedroom and store cupboard. 

The window to the right is a glazed dormer with a Brise Soleil (an architectural feature that reduces heat gain by deflecting sunlight) to help control solar gain in the summer.

7. Insulate your glass box extension well

seating area in glass-walled room

(Image credit: Paul Raeside)

Whichever type of structural glazing solution you are using, you need to consider solar gain. The heat from the sun’s rays can be utilised to heat the space, however with larger glass box extensions overheating in the summer can be a concern. 

If the extension is south facing, glass with a solar controlled coating is essential. This involves a metal oxide coating within the insulating cavity which reflects the sun’s rays and ensures the internal space maintains a comfortable living temperature all year round. 

It also takes away the worry of the rooms being cold in winter. You will hear the term ‘thermally broken structural glazing’ – this means the glass uses a low e-coating, thermal break technology with argon gas insulating cavities which results in the best thermal performance, which also helps the environment. It also means you don’t have to think about how to hang curtains which may detract from the contemporary look of the extension. 

8. Factor in the maintenance

When it comes to glass roofs or large glass box extensions, keeping it clean and clear is a concern. Ask your contractor about  a low maintenance coating that fills in microscopic divots in the glass resulting in an ultra-smooth surface that dirt and debris find it difficult to stick to. This really comes into its own in hard to reach areas. 

Even glazing is subject to trends and new ideas. As we have been spending so much time indoors in the last couple of years, floor-to-ceiling windows are popular, as are sliding glass doors that are moving walls of oversized glass. Structural glazing is characterized by its minimal design and flexible nature but this year architects and specifiers will move towards using glass as a structural element within buildings. 

Glass is now a key structural element  allowing residential designs to incorporate large elevations of minimally designed glazing even in curved and triple glazed panes.

‘As oversized glazing grows in popularity, and the temperature is rising, there is more demand for solar control and this should be taken into consideration at the start of the project,’ says Zachary Pulman, founder of Zachary Pulman Design Studio.    

‘Using solar coatings on glass will protect furniture from sun damage, as well as eliminate the risk of overheating. And think about investing in blinds, for privacy and security, including integral blinds in bi-fold doors which can be operated from a smart phone to open or close them.’

Do glass box extensions need planning permission?

Most glass extensions will require planning permission. ‘Although you can carry out some work under permitted development, it is best to have a professional assess the design and advise on the necessary approvals needed,’ says Ben Hawkins, Architect for Granit Architecture + Interiors. ‘A good architect will consider the aspect of the site, how much sunlight the extension will receive and how best to heat it. Proper specification of glass for the extension and working with a good glazing supplier is also essential.’ 

Are glass box extensions cold?

Glass box extensions can be cold, and also too hot in summer. 'Careful consideration should be given to the extent of glazing for both these reasons,’ says David Money of David Money Architects. ‘Roof windows give abundant daylight and you don’t always need to glaze the whole roof to create the desired effect.  You can also consider baffling the light by installing fins under the glazing.’

Alison Davidson

Alison Davidson is well-respected British interiors journalist, who has been the Homes Editor of Woman and Home magazine, and the Interiors Editor for House Beautiful. She regularly contributes to Livingetc, and many other titles, and often writes about kitchens, extensions, and decor ideas. She is the go-to for information about green energy, sustainable home improvement and eco design ideas.