A porch is exempt if:
the floor area does not exceed 30m
a door is retained between the porch and the rest of the house
there is no drainage within the porch and
glazing below 800mm in windows and 1,500mm in doors and sidelights is safety glazing.
You should also ensure that a porch does not cover outlets to boilers or other flues as well as the only ventilation openings to WC’s and other rooms.