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"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.
Even if you can't expand the demiplane, a 30 foot cube is large enough to hold a 3-story house.