I am currently modding using the towers of the Bard's college as add on's to my manor house.
Problem is, characters can walk through them.
The are lowered just slightly in the base terrain (maybe lowered 1/10 of their height).
I did not change scale.

-Are some buildings just missing collision detection?

I can probably hide a collision producing block of terrain inside, just wondering if this is common.
Any other solutions?

Thanks!

Probably. You can add collision planes...

Cool! is there a handy reference?
I assume this via the ck?

how/where do you add a collision plane on an exterior ?

Was able to add a collision cube. It is a gizmo button at the top right of the ck menu item / bar. Selected the building, selected the collision cube button, pulled the z-axis gizmo arrow; done! Worked like a charm! Just make sure you press M first, so you can see the marker box (yellow ). Verified it worked in game. Excellent !!