A caveat before anything else: the Building Regulations are revised regularly, they differ between England, Wales and Scotland, and the version that applies to your job depends on when the work is notified. Everything below is a map of the territory, not a substitute for your building control body or your installer's current guidance. If a specific number matters to your project, confirm it before you order.
The parts that touch a door
- Part L — conservation of fuel and power. Sets maximum U-values for replacement windows and doors.
- Part M — access. Where it applies, it drives the threshold detail.
- Part K — protection from collision and impact. Safety glazing in critical locations.
- Part Q — security. Applies to new dwellings rather than replacements in existing homes.
- Part F — ventilation. Mainly a windows issue, but relevant when doors are part of a wider replacement.
- Part B — fire. Matters if the opening is also an escape route.
Thresholds, and the fifteen millimetre question
An accessible threshold is generally understood as one no higher than about 15 mm above the finished level either side, with any exposed edges rounded or chamfered rather than square. Where accessibility requirements apply — principally new dwellings and new extensions — that is the target the threshold detail is designed to hit.
Manufacturers offer a range of threshold profiles to suit: a weathered threshold with a proper upstand for exposed elevations, and low or flush options where level access is wanted. The threshold options a fabricator offers are worth looking at before the slab levels are set, because the choice affects the screed depth and therefore the builder's programme.
Decide the threshold type before the floor slab is poured, not after. A flush threshold needs a recess and a drainage strategy designed in from the start. Retro-fitting level access into a finished floor means lifting screed, and that is a bad week.
Why flush thresholds leak, and how not to
A flush threshold works by removing the upstand that would otherwise stop water. Something else has to do that job. Where installations fail, it is almost always because nothing did.
The elements that make it work:
- A drainage channel immediately outside — a linear grated channel or a wide gravel strip, connected to somewhere that drains. Not a decorative slot.
- Falls away from the house across the terrace, in the region of 1:60 to 1:80. Flat paving in front of a flush threshold is asking for trouble.
- Correct damp-proof course detailing lapped into the threshold, with a proper cavity tray if there is a cavity.
- Clear drainage slots in the threshold itself, which need to stay clear for the life of the door.
- Enough clearance from finished paving so that surface water is never sitting at the same level as the door.
Much of southern Hertfordshire sits on shrink-swell clay, which drains slowly and moves seasonally. On a plot in Hertford or Cheshunt where the garden already puddles in winter, be sceptical about a flush threshold unless the drainage design is convincing. There is no shame in a 15 mm upstand that keeps your kitchen dry.
Safety glazing
Glass in critical locations — broadly, low-level glazing and glazing beside doors, within specified heights of floor level — must be safety glass, either toughened or laminated, and permanently marked as such. Any door leaf is a critical location, so the glass in a bifold or slider will be safety glass as standard. The mark is usually a small etched stamp in a corner; check it is there at handover.
Laminated is worth considering over toughened for a different reason. Toughened glass shatters into fragments when it fails; laminated holds together on its interlayer. Laminated also performs better acoustically and is harder to get through, which brings us to security.
Security
For new dwellings, Part Q requires doors and windows to be secure by design, which in practice means testing to a recognised standard such as PAS 24. Replacement doors in an existing home are not caught by Part Q, but there is no good reason to buy a door that would not pass it.
What to ask for: multipoint locking on the traffic door and shootbolts on the folding leaves; a cylinder to a recognised anti-snap standard; laminated glass in the accessible panes; and hinge bolts or equivalent. Ask whether the exact configuration you are ordering has been tested, because a system can be certified in one configuration and not another.
Notifying building control
Replacing a door or window is notifiable work in England. There are two routes: your installer is registered with a competent person scheme such as FENSA or CERTASS and self-certifies, or the work is notified to your local authority building control and inspected.
Either way you should end up with a certificate. Keep it. It will be asked for when you sell, and reconstructing it years later is tedious and occasionally impossible. If the doors are part of a larger extension, they are usually covered by the building control approval for that project instead — but confirm which route applies rather than assuming.
Conservation areas and Article 4 directions change what you can do to a front or visible elevation. Parts of St Albans, Hertford, Barnet and Enfield have them. Rear elevations in permitted-development extensions are usually straightforward, but if a door faces a street in a designated area, check with the council before ordering rather than after.
Listed buildings
If the building is listed, aluminium doors on a visible elevation are a conversation with the conservation officer rather than a purchase decision. It is not automatically a no, particularly on a modern rear extension to a listed house, but it needs consent and it needs to be asked before anything is ordered.
Common questions
What is the maximum height for an accessible threshold?
Around 15 mm above finished level either side, with edges rounded or chamfered rather than square. Confirm the current requirement with your building control body, since guidance is revised periodically.
Do I need building regulations approval to replace patio doors?
In England, replacing a door is notifiable work. Either your installer self-certifies through a competent person scheme such as FENSA or CERTASS, or the work is notified to local authority building control. Keep the certificate for when you sell.
Is a flush threshold a bad idea in a wet garden?
It can be, unless the drainage in front of it is properly designed — a channel or gravel strip, falls running away from the house, and correct damp-proof detailing. On heavy clay, a small upstand is often the wiser choice.
Does glass in a bifold door have to be toughened?
It has to be safety glass, which means toughened or laminated, and permanently marked. Laminated is often the better choice because it holds together when broken and performs better acoustically.