Extending Interiors to the Outdoors

ABOVE: The huge sliding glass doors on this remodelled 1930s terrace mean that the ample garden seamlessly seeps into the room and light effortlessly bathes the open plan space. Read more about this project

Take the concept of traditional French doors and apply the contemporary design ethos of minimal structure, maximum light and you end up with a door opening that extends from floor to ceiling and the full width of the wall, with frameless glass doors. Lay the same flooring inside and out and make sure the threshold is level and, whether the doors are open or closed, you have a seamless flow between indoor and outdoor living areas. Removing the visual boundary between inside and out creates a remarkable sense of space and in the summer months, when the doors are open, the two spaces can be used as one.

This is the ideal, and when done well it looks incredibly simple. However, frameless glazing is expensive and level thresholds are quite complicated to create. Fortunately, if you are willing to accept a degree of compromise, much the same effect can be created relatively inexpensively.

Design options

The biggest decision to make is the type of door best suited to your needs and budget. Many people install a series of French doors that open out to the garden. The problem with this style of door is that there would be a frame on every set, which wouldn’t allow for one wide open space. Sliding doors give a clearer, more unobstructed view of outside through large panes of glass, but still cannot open the whole stretch of the space, as one half simply slides over the other. The best way to get a fully open space is to install folding sliding doors, which fold back onto themselves, enabling almost the whole wall to be opened — up to 90 per cent.

The next decision to make is whether you want metal, PVCu, timber framed or frameless glass doors. Timber is ideal for older, more traditional houses. It has a great natural look, but some upkeep is required and the wood will need staining every couple of years. At the opposite end of the scale, metal doors, such as aluminium, are more suitable for a contemporary property and don’t require as much maintenance. PVCu is generally the cheapest option, however, it can limit the amount of glazed space and is prone to movement caused by temperature changes. Frameless glass doors are visually the most effective as they are all but invisible, however they are also by far the most expensive option.

What are the structural implications?

Door systems are not designed to play a structural role and so a suitable lintel must be installed over the door opening to replace the section of wall being removed in supporting the structure above. The standard solution is to insert a steel joist, possibly designed with a flange to carry external brickwork. The length and size of the steel and the details of the bearings will need to be calculated by a structural engineer. When the opening is created, the wall is knocked through in sections, and the structure above temporarily supported using acroprops.

How do I satisfy the Building Regulations?

Having a suitable means of support above the door is not the only criteria you have to meet to satisfy the Building Regulations. Glazing is generally a source of heat loss and this is an important issue. In order to meet the Regulations, glazing will have to achieve a U-value of 2.0 W/m²k for PVCu or timber frames and 2.2 W/m²k for metal frames. However, if the glazed area of your house exceeds 25 per cent of the overall floor area, there are more criteria that need to be met. In this instance you should seek guidance from your local authority building control department.

To minimise U-values, specify low-emissivity glass, which reflects the heat back into the home. If you are opting for aluminium frames, make sure they have a thermal break to ensure your system is as thermally efficient as possible.

Bear in mind that you will be installing a large amount of glass and, as the glazing will cover what are termed as ‘critical locations’, toughened, laminated or reinforced glass must be used for safety purposes.

Something you may not have thought about is air circulation. Existing means for background and natural ventilation must be retained. If you remove a vent, it needs to go somewhere else and open flue appliances may need to be checked over.

You will need to arrange building regulations approval yourself, unless you employ a contractor or installer registered under the FENSA (Fenestration Self-Assessment, fensa.org.uk) scheme.

Do I need a structural engineer?

Yes. They will calculate the current floor, ceiling and roof loads and choose the best materials at the right size for the job, ensuring that the wall will have the strength to perform its function safely and won’t sag or be in danger of collapsing. Most specialist companies will have a structural engineer on board, so if you do hire a company, find out from them if it is part of the service. Alternatively, to find an independent structural engineer, check out findanengineer.com. All the companies listed on this site employ at least one Chartered Member of IStructE (The Institution of Structural Engineers, istructe.org).

Do I need planning permission?

Not unless one of the following circumstances applies: firstly if the right to install new doors has been removed by a planning condition attached to a grant of permission on the house, or, secondly, if the house is subject to an Article 4 direction to prevent new door and window openings being put in. Otherwise, planning permission is not needed unless the building is statutorily protected e.g. listed.

It is worth noting that the Regulations are currently under review, so if you are not building for a while check with your planning office.

How much will the doors cost?

It is difficult to specify an exact price, as companies will quote each project individually. This is not a cheap solution, however. As a very rough guide, a 150m² detached home with a standard wall of 4.5m wide would cost around £5,000, including installation and VAT (or more for frameless glass doors). In general you should expect to pay a minimum of £1,000 per panel or door set, fully fitted. The higher the specifications for the project, the more expensive. Always get quotes from several reputable companies before making your final decision.

A level threshold

Ideally the doors would open out onto a level patio, using the same flooring, to create a seamless flow between outside and in. If the garden floor isn’t level, decking can be installed, or (more drastically) the garden could be landscaped to level it out. However, if this isn’t an option, then a step onto the garden is the remaining answer.

Level or not, the doors will need to be waterproofed and a drainage detail will be needed in front of the door, such as a sunken channel along the length of the doors to direct water away from the house.

The doors themselves will need to be watertight as far as is possible. It would be a good idea if the doors were designed with some sort of weatherboard and sliding or folding doors will need a flush or weathered track. Take advice from your suppliers.

Further reading:

• Contemporary Living in an Old Home
• A Buyer's Guide to Patio, Sliding and French Doors
• How to Achieve Instant Character