I don't think it's safe to presume anything! If you're renting, the owner should know. If you, or the other people, own the flat, it'll be on the plan at the Land Registry. We used to rent in a similar situation, and the garden was used only by the ground floor flat, even though we could get to it. We own a house now, and our neighbours have access to their front door and garden across our land - a fact we knew before we bought - and it's clearly marked on the plan at the Registry. What do your downstairs neighbours think? Maybe if you're both unsure, and both rent, you could come to some agreement??