Have you thought of wanting to expand your house and make it bigger?
Add more rooms, more items and what not?

Time to start with Cellarage Expansions.
The expansions will only be as big as the yard itself when outside the house, each divided up.
Say a Small house is in North-West corner, there will be 3 expansions to the Cellarage, which would be beneath the yard itself.

Medium and Large houses would have more expansions - say 5 and 7 respectively.

What these expansions would do is give players more room to decorate their houses, and possibly increase the amount of items you can store in each house universally - so regardless of S, M or L house, you get same item storage increase per expansion. To keep it simple, each expansion increases item storage by 30, so Small would be +90, Medium would be +150 and Large would be +250 items - added from base housing item amount, when all expansions are brought.

Expanding the Cellar is just the beginning. Imagine adding additional rooms via some device created from Omega that you can add to your house so you can technically create infinite rooms - similar to how a FC can have 200 rooms, per member, but this would be on a larger scale - say a hallway is added as its own instance inside the house, any room attached is also part of the hallway instance, and there's a door leading back to main building. This would pave way for expandable housing, and perhaps introduce a room limit too as not to expand too big - Small 6 rooms, Medium 15 rooms, Large 40 rooms.