Personal houses are just for show. You can have an FC chest in it, even if it's a personal house but that would be shared by your FC. The only real way to up personal storage is to buy more retainers for real money through the mogstation.

If you're wanting a personal house, which it seems like you're not, you can always check https://www.reddit.com/r/FFXIVhousingmarket/ where people sell them. Or the party finder constantly has houses for sale. If none are being offered you can make a party finder with WTB house.