Be careful when replacing with a uPVC door.
Part M sets the maximum threshold height at 15mm
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1655,en.pdf
Access into a Dwelling
1.23 Where the approach to a dwelling entrance
is level, or sloped, i.e. free of steps, or, in the case of
apartment blocks or the like, is provided with a
suitable passenger lift in accordance with par. 1.12,
and there is at least one habitable room at entry
level, the entrance should be suitable for wheelchair
access. In this case,
(a) the entrance should be provided with a level
entry, i.e. with maximum threshold height of
15 mm, and
(b) the minimum clear opening width of the
entrance door should be 775 mm.
You will see this 15mm limit mentioned on this web site:
[broken link removed]
I have found some uPVC suppliers are providing items that are more like windows than doors.
They have full frame surrounds, with huge bottom frame members/upstands of 75mm.
Its all very well have a well sealed door, but there's no point breaking your neck.
You might be better advised to install a porch instead of a uPVC door.
As long as the area is 2.0M or below, it is exempted development.
It provides an air-lock effect which even a sealed door cannot.
FWIW
ONQ.