Runkulam
Simple (hah, I know the worst thing you can say to a programmer is "Oh, it's simple!") solution is to make two levels of apartments/guilds halls/etc. Keep the first level as they are but offer other levels with 2x, 3x or 4x the storage space. Have recurring rental fees based on the extra storage.
Would you expect that the apartment be physically larger as well? Storage space can be accomplished pretty easily and I believe through sheet configuration. The code challege is adding a system in to handle recurring in-game payments.
If you wanted the space to be physically larger there would be artistic assets involved which is arguably more expensive to accomplish than the code.