Use Sketchup to create a 3D model of a building.
If you only have 2D images/pictures, you start using that as your base to create 3D models.
Install the Extension "sketchup-stl-2.1.6.rbz" via: Window => Extension Manager => Button "Install Extension" and select file.
Settings: Menu item "Window" => "Preferences" => Select "General" unmark "Allow checking for Updates"
After you have editted a drawing do a purge of unused stuff via: "Window" => "Model info" => Select "Statistics" => Click on "Purge Unused" AND => File => Save.
After the installation of the Extension "sketchup-stl-2.1.6.rbz" you will have a new menu item below menu item File named "Export STL...".
Download sketchup-stl-2.1.6.rbz via: https://www.unix4life.com/downloads/sketchup-stl-2.1.6.rbz