I'd just be sure that they and you have the same ideas about the garden. They may be thinking that you'll make them a nice pretty garden that they can enjoy looking at and sitting out in while you look after it for them, all free of charge; while you may be seeing it as an extension of your own growing space where you can have overflow pots and suchlike. Also, be aware that anything you put on their property will be out of your immediate control and could get damaged or 'disappear', eg if they have parties, barbecues etc.
