Bifold doors also called folding patio doors, bifolding doors, folding sliding doors, glass sliding doors, sliding room dividers, internal folding doors, concertina doors and uPVC bifold patio doors.

Bifold doors will cost between £1,950 and £3,900, depending on how many glass panels they have and the colour/style of the door frame.

The more work the job involves, the more installation will cost: so a straightforward replacement of existing patio doors will cost less than expanding a doorway or window or creating a new opening – all of which involve a lot more work, and for which you will need a structural engineer (to make sure the weight of the doors will be properly supported) and a builder. A builder will probably charge between £150 and £240 per day; consulting a structural engineer should cost around £100.

You can design your bifold doors however you want. For example, a 4-panel door could open at one side and have all 4 panels sliding the same way. You can choose the arrangement that suits you best and makes best use of the space you have. You’ll need to have space for the panels to stack at 90 degrees when the door is open, and you can choose for them to stack either inwards into the room or outwards into the garden.

The larger the panels, the more glass and the less frame you’ll have, so the more light will come into your home and the better your view will be.

Look closely at the company’s history, warranties and the quality of their products. Make sure the firm has public liability insurance and is a member of a competent person scheme.

Finger-safe gaskets and childproof locks for child safety.

Locks: look for deadlocks (so no one can get in without a key), internal shoot bolts that lock into the frames by at least 24mm, hook locks, 5-lever mortice locks and cylinder locks that are protected against snapping, drilling, picking and bumping. Make sure there are multiple locks along the tracks.

Safety glass: either toughened glass, which is very difficult to break, and if it does break, crumbles into small chunks designed not to cause injury; or laminated glass, which stays in the frame if it is broken. By law, a door with glass 1.5 metres or less from the ground must have toughened or laminated glass.

Internal glazing and beading: this means the glass can’t be removed from outside.

Concealed hinges, locks and rollers: to ensure an intruder can’t tamper with them. Cheaper uPVC bifold doors may have exposed hinges on the outside; these are a security risk as it’s then possible for an intruder to take the door off.

Security screen doors: these are mesh doors that can be installed behind or in front of your bifold doors, so you can have them open to let fresh air in but still prevent anyone getting in.