Suggestion: Upgrading Apartments

It gets mentioned in almost every housing thread and I think it's a big enough topic to get its own thread. Improvements and Upgrades for Apartments!

What do I mean by Improvements and Upgrades?
Being able to expand the size and/or item limit in Apartments, maybe even adding a loft/second story.
It could be done in phases and be a gil-sink.

As an example:
We start out with the vanilla apartments we have now.
Phase one could be expanding the item and storehouse limits to 200 (the same as a small house). Cost 500k.
Phase two could be either expanding the ground floor to the size of a medium house's basement OR adding a loft/second story (while also increasing the limits to 300). Cost 2.5 mil.
Phase three could be doing the other upgrade you didn't do previously. Cost 15 mil.
Phase four could be the final one and let us upgrade to large house's basement size (and increasing the limits to 400). Cost 30 mil.
I tried to make the upgrade cost close to purchasing an actual house of that size when you add all the cost together.
Maybe as a restriction, people who start to upgrade their apartments can't own homes and vice-versa (owning a vanilla apartment will be fine, like their grandfathered in).
(You could even ditch the third and fourth phases and I still think it could work)

Why do this?
To help with the limited stock of houses and how annoying it is to buy a previously owned plot. The pay off would be that you won't have an outdoor area and that you will need to pay for upgrades.