The lease is ambiguous. It technically says that the whole house is rented exclusively to 3 different ppl. However, you can't claim the whole house because the other people were already living there so this is clearly not what was intended. Based on what you said you were given enough information to be able to conclude you may have no rights to the garage. The argument that you would not have bought the bike if you thought you had no rights would not fly IMO because if something was not clearly specified you should have clarified it before buying the bike. Also if your lease does not specifically state garage, to me, this means no garage. The landlord is keeping it for himself. My units have storage lockers and my leases state that the storage locker is rented. BUT this said, I'm not the judge. I just don't see it flying though. But even if it did, you'll be living with your roommates in a "divorce" atmosphere. You might as well move!

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Also when you went to see the room before renting it: Did anyone show you the garage and say, "And this is the garage" for example. If so, this could help you establish that this was something that would be available to you. But if they just showed you the room, you probably just got the room.