Home.by.me is pretty decent in my experience. I have tried a few free ones out and that one has been the easiest to use but that ease of use does come at the cost of versitility. So if you are trying to make some avant garde concept, not really possible. Like I don't think it can do a curved wall IIRC.

It can be useful to use to think about things and mess around. It might have some value if you designed a home in it and then brought it as an example to help your builder and architect understand your vision so they can actualize your housing dreams envisioned under lithium and drawn on a napkin with a crayon in a closed fist via JavaScript.

Designing and building from scratch is considerably more expensive and a longer process than modifying an existing blueprint, but we are talking about dreaming here.