So is it like gimp, where you can draw on different layers and stack them, or doing the whole plot, textured grass, then drawing the house with suitable flooring etc, simply overly the plot textures?

The twice ! [;)]
So it's like a lego game. [:)]
You choose the ground, after the bricks for the walls, ...

Layer in sh3d is named level (to separate the height, example: cellar, first level, 1st floor, roof, ...) but it's an option.

The best of that is the practice. [;)]

[:)]