IT varies depending on each charity CC - I dont have a garden, but I have a spare room that has been made into a 'cat room' rather than pens. Mine are isolated for the first two weeks, then whehter they come out depends on them really - Finn has only had 2 periods out as he walks round my house yowling, yet is fine in his room (until you leave him, as he wants to be with you all the time - but let him out, and he ignores you!!). You need to have some way of isolating cats in case they do have something, or if they are aggressive towards your own.