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All characters on the same account are counted as the house owner and have the same access as the character who's name is on the house sign.
it works also if you have a Vendor in said house, all your characters have access to the vendor to place and remove items and gold from vendor.
I per-say have 2 accts, one being the house owner (just that), and the other one for the Co-owner status. Only pain is that when I place something I do have to choose the Co-owner status so other characters can access it, otherwise I can't pick it up or remove it, unless I log in to the character that placed it in the beginning.
For the house storage slots, I made sure the 1st and 2nd accts have the 60% storage increase to maximize the space as needed.