In the case of David Cliff, it's so big that it should be safe to say you can claim a section of roof for a garden without it affecting other player's ability to also use other sections of the roof or for it to affect their immediate living spaces. The roofs are connected so you don't have to worry too much about the placement of the garden on the roof.

If you'd like, once it's completed, you can comment in the new housing section that there is now a roof garden on part of the roof and I can link to a page about it on the spreadsheet if you want to make one.