Another reasonably simple tutorial for attaching a garage to a house with a foundation.
https://www.youtube.com/watch?v=ZNPLFwVV_zg- Uses constrainfloorelevation (true/false)
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Using CFE to Add Doors to a House with Foundation✽ Note: It maybe best to do this while building the house instead of trying to add on after the house shell is fully finished.
1. Press Ctrl Shift C together -- Type in testingcheatsenabled on and then type in constrainfloorelevation false (Turns CFE on) and press Enter.
2. Draw the foundation wall in from the ground level around the back door area.

++ It will spike up on the garage side of the house and need to be fixed by leveling the terrain from the 2nd floor/roofline and the foundation level of the house.

3. Draw out the room walls around the back door area and the rest of the house, if not done yet.

4. Use the sledgehammer tool to delete 2 - 4 wall pieces in the foundation wall that the door (s) will be attached to.

5. Draw back in the walls at ground level in where you'd like to put the door (s).

6. Add the door. Making sure it does not have a grid by it when trying to place.

7. Add free wood flooring or level the terrain from the 2nd floor/roofline level of the house.

8. If all went well, you should be done with CFE and can safely turn it off. Press Ctrl Shift C together -- Type in constrainfloorelevation true (Turns CFE off) and press Enter.

-- Finish up the build and decorate/furnish to your liking.
