Houses are per-character. If you already own your maximum amount of house's on your character you can purchase additional houses here on Tebex. If you do not own a house and purchase a Tebex house, it will contribute towards the limit of houses you can buy with in-game currency - i.e. Once you own your maximum amount of house's, you can only get additional houses for that character through Tebex.
You can unlock more house slots by having a subscription.