Once I finish this pancake I will have had 5 pancakes.
Would you kindly explain this kind of future perfect?
It's a sort of future retrospective tense, where one is talking about a past tense that has not yet occurred, but is expected to occur. In the case you asked about, it introduces the information that the building does not yet exist, or that some other related event is still yet to take place in the future.
So why do we not say the following instead of the sentence above?
Once I finish this pancake I will have 5 pancakes