Missions

You don't always unlock quests/buildings on the levels the descriptions say they're available at.

In actuality, the levels mentioned in the building descriptions are the earliest possible level they can show up in. Often, quests and as a result buildings can get delayed because people assign characters to tasks outside of the quest line which they then have to let end before it will trigger the next quest.

It is not unusual to be a few levels above the minimum level required to unlock a building before you finally unlock. I was always a couple levels ahead of my building until I hit the last level.