Hello Christophe,
Many thanks for coming back! Unfortunately, we need to keep different zones as zones in order to be listed in different types of schedules ( it is a complex project on an existing building so we need to show many different things in schedules, like which area is existing compared to proposed, which area is now Flat 1 or 2 etc which is a different zone, then schedules of objects etc). I tried making some zones 0 height hoping that they will not be recognised but it did not work... I do not know maybe I do something wrong there.
Thank you anyway, useful to know what other users do.