I like these ideas, but they'd probably increase the cost on apartments...

So lets tweak this.

What if all those things were options that could be bought. Double the cost to double the size, once you purchase it for the base cost.

Add in another 500k for the backyard door.

Another 500k to double storage, inside or outside.

Once you hit a total of 5 million spent, it should basically be a small house worth.

For the next set of upgrades, make them 5 million each, so at 25 million you've got a medium house worth in your apartment, spent and recieved.

And of course, push it to 50 million, 100 million, 500 million.

As long as people have to pass through a DOOR to reach the new space, instancing can happen properly, people can be split and shifted as needed.

In the end you'll end up with a space equal to a large, with less server demand then an actual large due to the limited amount of it that will be loaded at once.

Door into a stairwell, climb the stairwell and go out a door to the floor you want to be in. Only ever one floor loaded per apartment per person.