The minimum clear width of entrance doors to new dwellings are:
Category 1 dwelling - 775mm
Category 2 dwelling - 850mm
Category 3 dwellings - 850mm
Below is an extract from Approved Document M Volume 1 showing how to measure the clear width of a door
A Category 2 dwelling should have a 300mm nib to the leading edge of the door, i.e. the handle side, in the direction of opening.
A Category 3 dwelling should have a 300mm nib to the leading edge and a 200mm nib to the following edge. The following edge is at the handle side of the door to the edge that the door opens away from.