You have to buy the "Bric-A-Brac" elixir in the crowns shop. It costs 995 Crowns. Make sure if you want to increase the inside of your house that you're stood inside, or it will increase the amount of items you want in your garden, and vice versa.
I agree one hundred percent. I always end up purchasing the elixirs for my houses, but I still struggle to decorate to my happiness. Especially in the houses that have a lot of surface area and a lot of decorating potential (Botanical Gardens, The Acropolis, Darkmoor Manor, even the Royal Estate from Marleybone), I find that having a max limit of 350 items takes some creative juggling to get everything just right.
Squeezing in the time to decorate each house is a hobby of mine, and I can get really into the flow. Then I get this pop-up box saying "You cannot place any more items" and simply weep in despair. I've had to completely redo multiple castles to get things to fit just right. Then things sometimes end up looking a little sparse.
I have every single house in wizarding history (except for Treetop Getaway and Watchtower Hall, and the apartments from Mirage as I have yet to get there. I will get them though!), with a current total of 44 castles. I am an intensive collector, so my attic always has 600+ items in it, my banks almost always get maxed out, and when there are less than a hundred items in my backpack, I practically cry in bliss.
To be honest, I think the way things take up space in our backpack ought to be changed. Why do seeds take up a slot? I think they should be like jewels, with their own little space like a gardening bag or something. And I recently made a post about the housing duel challenges. It would make a bit more sense to have a housing duel system, where you can input the duels you get and not take up huge spots in your houses. I can put down a total of 39 in the Massive Fantasy Palace (my current Duel and Trophy house), but it looks incredibly bulky so I tended up trashing a bunch. /cries
But to get back off this tangent, I really do agree that the bank and houses could seriously use an upgrade. I will always be happy when KingsIsle increases limits (whether it be banks, backpacks or houses), but there is always room for more space!
You have to buy the "Bric-A-Brac" elixir in the crowns shop. It costs 995 Crowns. Make sure if you want to increase the inside of your house that you're stood inside, or it will increase the amount of items you want in your garden, and vice versa.
I wasn't clear enough then. Even with the elixir you can have a max of 350 items inside and outside. It might be a lot but about 450-500 should be the limit.