Would you rather have a system where you 'build as you go', similar to Fallout 4's system and haven's system where you build one post/wall/floor/door segment at a time? Or as Goo suggested a bare bones 2d base schematic when you mark walls/floors/doors and generate a "blueprint". Creating a whole 3d CAD type UI is way too much work, but it would allow for the greatest level of customization.
I feel the build as you go method would work well, but then you would need a whole menu for house parts which can get cumbersome, and you wouldnt be able to simple dump all your material into a construction signpost. (this could however be a good thing if you want the utmost variable material variety, build each segment seperately with which material you want)
2d blueprint seems the easiest way to create whole buildings using one signpost, with each floor being one layer of blueprint. variable material might be difficult if you want each wall/floor/door to be different
3d CAD UI would be crazy, but also kind of breaks immersion
Two ways of making buildings: One is free for like you said, built all individual like, but second can be "previewed" on a sketchbook page thing from the shop, and can be used to build the house in a little imaginary little side thing, then you can place a building sign for it with a certain amount of uses.
At the end of the day you could do with a full 3d modeling program if you really wanted freedom. Not sure what the right level is.
"The psychological trials of dwellers in the last times will be equal to the physical trials of the martyrs. In order to face these trials we must be living in a different world."
I really don't enjoy having to design the building outside of haven to place it in-game, like a custom sketch. I'm much more interested by in-game building, beam by beam, pillar by pillar, with a simplified system of weight distribution of resistance of materials.