A threshold is a drainage decision first
The threshold argument usually starts as an accessibility question and ends as a water question. A weathered upstand puts a raised section under the door that water has to climb before it can get in. A low or level threshold takes that step away, which is better to walk over, push a pram across or wheel a chair through, and gives the water less to climb.
Neither is the right answer on its own. The correct one depends on how exposed the elevation is, what falls away from the door outside, and whether there is anywhere for water to go once it arrives.
That is why the question belongs on the drawing rather than in the showroom. On a sheltered rear elevation with a fall away from the door and a drainage channel, level access is straightforward. On a west-facing elevation with driven rain and a patio that sits flush, an upstand earns its place.
What each option asks of the build
Level access is a build decision as much as a door decision. It needs a fall away from the opening, usually a drainage channel immediately outside, and a floor build-up that lets the finished internal level meet the outside without a step. Those are all things the builder sets before the doors arrive, and none of them can be retrofitted cheaply.
A weathered upstand asks much less of the build. It tolerates a flatter patio and less drainage, which is why it remains the default on exposed elevations and on jobs where the external levels are already fixed.
Our bifold route carries a 20 mm thermally broken low-threshold option where the detail suits it, and sliding doors are specified with the threshold preference alongside the track layout. Tell us which way the job is going at quote and the threshold is quoted with the rest of the unit rather than changed later.
Where accessibility comes into it
Approved Document M covers access to and use of buildings, and level thresholds are the usual route to an accessible entrance where it applies. Whether it applies to your job depends on the project and the work being done. Our post on building regulations checks for trade glazing sets out how the regulation questions tend to arrive together.
Where accessibility is in scope it is worth settling the threshold at the same time as the structural opening, because the two constrain each other. A level threshold that is decided late tends to mean either a step nobody wanted or a drainage detail improvised on site.
What to send so it is priced once
The structural opening, the elevation it faces, the external finish and its falls, and whether level access is required. If there is a drainage channel, say so; if the patio is already laid, say that too, because it removes options.
Send those with the drawings through quote, or read the configuration detail on the bifold and sliding door pages.