Shows up as an internal map for me but could be faked, however does not get this loading times other places either. Note that I'm not an decorator, everything I brought in is in containers.
Anyway my point is that the players house and settlement is resource heavy to handle. If game can handle it it can also handle normal structures who are much larger.
I think that major structures get internal cells, small houses don't
And its ways to work around this, by messing around with the loading system, skyrim has the system where you run into an cave and it loads, now if the exterior cell was scripted to also load the cave or load it then close to opening you would be able to enter with no load delay, yes it would still be an internal cell.
No this would not work well with an highrise building with windows, it will work for caves, subways and vaults.