I just wanted to add that for the quest "Have a Clan House", it is possible to put down the Clan House (doesn't need to be connected to a road) and the quest will complete. The Clan House can then be deleted and the quest stays completed.
However, the same is also true of the last quest to "Have 1200 Diplomacy" AND "Have 6 Cultural goods buildings" in that if the Diplomacy sub-quest is completed, then building the 6 cultural goods buildings will complete the quest (not just the sub-quest). At that point, a goods building can be deleted and the quest stays completed.
From that perspective, there seems to be a difference between completing a quest and completing a sub-quest. A quest, once completed, always stays completed, while there are times when a sub-quest can be completed and then become uncompleted.