Are there planets that lack mountains or have constrained availablilty of solids. For example a planet that has clay but not more solid stones like normal stones. The direct reason why I wonder is because I started to wonder if there existed planets where one would have to build less solid houses then at the earth due to the solids available if a person would build a house on that planet hypothetically spoken. It would be really nice with a detailed answer of what solids another planet would lack and how it constraints the housings that could be built on that planet in terms of solidness.
A planet has been theorized that forms in a primordial disk of high carbon content. See https://en.wikipedia.org/wiki/Carbon_planet. This planet's crust would have a high hydrocarbon content and thus not be very "solid". However, such a planet might theoretically have volcanoes that erupt diamonds, so an alien could build there home out of diamond!
Another hypothetical planet is a waterworld or ocean planet. If there is indigenous life then the only "above water surface" would be composed of life (ie: mats of algae type material floating on the sea).
As far as mountains go, this is not a question of solid or not solid. A large superearth would likely not have much in the way of mountains because of the high gravity at the surface. It might be noted that with out plate tectonics, we wouldn't have mountains and the Earth could arguably be covered in a global ocean.