Following up to my previous post, which discusses what kinds of fruit might be suitable for people to live in, I will now address Rebecca’s comment about the possibility of using giant walnut shells, which have interior divisions already, and so might not need more walls built.
It’s true that walnut shell is a good sturdy material, and if you can get it to grow big enough, it should work nicely as a home exterior and (as noted) interior. It might be a challenge to get it that big, though. Walnuts grow on trees, and so either the branch has to be big enough support its weight, or you have to build and adjust a scaffolding under it while it grows. My preferred alternative, pumpkins, are much larger to start with — I’ve seen ones big enough to make a doghouse, at least — and grow sitting on the ground. Plus, it’s more fun to say “pumpkin.” Say it several times. Don’t you just have to smile?
As I mentioned before, one of the challenges for your environment-friendly gourd house is finding compatible furniture. Maybe we can’t get a walnut to grow big enough to live in (within the limits of known Science), but I bet we could get it big enough to sit in.