I FIGURED IT OUT!!!
Click on what you’ve built, it will show you the arrow and building height, click the “shared” dropdown and change it to the address of the lot. Then you’ll be able to build as you please.
Just discovered this by accident while also looking for an answer to this problem.
What about staircases that are a part of two-story foyers? I frequently can't add the second story part of the foyer to any room - I managed it one time in the attached screenshot to assign it to a unit, but not have had any success with the other units. Therefore, I can't use the workaround, and I am getting an error whenever fences or walls touch shared walls (see attached, highlighted in yellow). The fence is in the unit I am editing, but it is telling me it is not part of the unit.
It works, ly.
