I created an example for you to study:
The basement is what you would use for the shooting range. It's just an underground room: level elevation -270. You can enlarge it outside the ground level house walls and of course make the walls thicker. Put a room around the house at level 0 that covers the part that goes outside the house perimeter. That is the ceiling for the basement part that is not under the house itself.
The middle room is partially sunken into the ground to show how it works with the levels.
The garage is also an underground room. The slope is a single wide sloping wall but you could also use a triangle object or create a shape. (Another alternative is using the Terrain Generator.)
The light blue floor and the wall around it is underground and just to show the outside of the underground parts. If you make the indicated level invisible the underground parts disappears from view and the underground parts are hidden below ground.
With this example you can see what different level heights do and where you should adjust level and wall heights to fit your need.
You can download the project here: https://dodecagon.nl/upload/tmp/LevelsExample.sh3d
----------------------------------------
Dodecagon.nl
1300+ 3D models, manuals, and projects
