And yet for some reason many people think that the demiplane can't grow this way: http://www.enworld.org/forum/showthread.php?657678-Demiplane-walls
I think the language is ambiguous: "When opened, the door leads to a demiplane that appears to be an empty room 30 feet in each dimension, made of wood or stone." Emphasis mine; the spell doesn't actually say what the demiplane is made of or even how big it is, only how it appears. That's a really weird way to phrase a spell.
Basically, it comes down to "It doesn't say you can, so you can't" which is some combination of "doing it wrong" and "no fun allowed"
Even if you can't expand the demiplane, a 30 foot cube is large enough to hold a 3-story house.