The AVMA advises against fixed space and exercise requirements, because a dog's needs vary. So the question is not your floor area. It is your routine.
Measure your building's round trip in minutes before you build a schedule. Why predictability beats frequency, and how to make a trip 30 seconds of prep.
A pad in 40 square metres is not the same problem as a pad in a house. Placement, a tray, and a 24-hour change do most of the work. Plus the trade-off.