It isn't something you can officially 'own' without buying all the houses there and even then you will only 'officially' own the doors the houses are placed at and the interior, not the entire floor. If it's an illegal group you plan on creating, you can own it with roleplay (fear, turf etc).
And by 'empty' you mean there are no houses on that floor to purchase, you must donate and you will receive a
houseclaim which you can use to put a house there.
Bojan_Smith wrote:how much is it?
Dont ower do it.
Contact the house owner(s) for that information. It's their property and price is only known to them.