I took a good look at some of the chairs he used and I can't be too sure. On the one side, they look a bit wonky, and other hand, they're really well proportioned. I'll have a closer gander shortly.
I did recognize a crane arm in one of the basements, which looks very much like it came from a plastic toy crane. There might be other stuff in there that came from toys or dollhouses.
Wheeh; now it's not just gawking at the madness/genius, but also a searching game!
+++EDIT+++ Yup; I'd say that the bath and toilet in the top picture are dollhouse furniture, as well as (probably) some of the lockers [basement of the first picture on the left, first row], the sink and tub in the second one and the chair in the submarine picture as well and probably a lot more.
Still; it's an awesome piece of work (all of them), especially since all those books, ledgers, files and binders appear to be handmade.
Definitaly a source of inspiration; I've already got thiis bookmarked!