Before my daughter was old enough to play with real Legos (i.e. not Duplo ;) I hadn't thought about the interior so much. And I thought that she could just disassemble the modular into sections if she wanted to play with them. Well, you live and you learn.
Can your hand fit in a roofless modular Lego building? Yes.
Can you play with mini-figures, accessories and furniture inside a modular room with four walls? With much difficulty.
Solution? You knock down the back wall - or even better: build the building without it. This is in fact a two fold winner strategy. A: Your kid gets much better space to maneuvre inside the room and B: You get lots of left over Lego bricks. Win win :)
I am pretty sure that even when my daughter no longer wants to play with Lego (and only build) I will continue to build like this. I can see inside the buildings without pulling off roofs and sections and it is way more economic brick wise.