Let's put it simple:
You can have mulitple houses with multiple item limits. You can invite peope to your houses, when they enter item data is loaded.
You can't have ONE house with very high item limit - but you can have huge houses with alot more item limit compared to small houses.
So, why you can't have a small/medium house with the item limit of mansions? Or ONE house with higher item limit.
And then we have the free version vs paid.
The solution is simple. High item limit per account, not per house. Won't hurt the server.