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

• 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. Is this a quote from somewhere? Why are you talking about buildings and using the second person? Commented Apr 11, 2014 at 14:02

Okay, I'll take this context in mind and try to explain building a story.

Will have had simply means that in the future, you'll finish/have something in past! If that makes you confused, let me simplify.

Once I finish this pancake I will have had five pancakes.

It's 5 o'clock in the evening. And up till now you have already finished four pancakes. So, if someone asks you now, how many pancakes you had (up till 5 o'clock)? You'll simply reply - four.

Now, you cannot resist that mouthwatering dish and it's being baked. You'll then say... Once I finish this pancake (pointing at the oven, maybe at 1730 hr), I'll have had five pancakes.

I will - future (1730 hr); have had - go in future and come back as you already finished 4+1=5 pancakes (total pancakes till 1730 hr).

• All your answers are fantabulous and priceless for me. Thank you all so much.
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Commented Apr 6, 2014 at 15:06
• I can say certainly the fact that I in the whole got the whole you have taught me.
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Commented Apr 6, 2014 at 15:07
• @nima_persian you are welcome. We are here to help enthusiasts like you. However, try not to repeat the question. Commented Apr 6, 2014 at 15:52

The meaning would be different for each sentence. The difference turns on the various meanings of “have”.

Once I finish this pancake I will have 5 pancakes.

Here I would assume you are making pancakes, and soon you will have made five of them. Having here means possessing – they are sitting on a plate next to the griddle.

Once I finish this pancake I will have had 5 pancakes.

Here, on the other hand, people would understand that you are eating pancakes and soon will have consumed five of them. Having a pancake and eating it are synonymous in this case.

Are you familiar with the saying about having your cake and eating it too?

• This should be the accepted answer. Commented Jan 13, 2022 at 4:52
• Yes, the key point is that "have" can mean "eat something (for a meal)" Commented Jul 23 at 14:31

This would be easier to answer if you say what "finish" and "have" refer to. At least it could refer to "eating or "making". You can add these verbs to the sentences, which would help make the meaning clear.

Anyway, as they are:

The first example really only works with eating ("had" does not imply making). The second example could refer to eating or making with different interpretations.

So

Once I finish this pancake I will have had 5 pancakes

means you have eaten four pancakes up to that point, and after finishing "this" pancake you will have eaten five.

Once I finish this pancake I will have 5 pancakes.

Referring to eating, it means once you finish "this" pancake" you will have five more. "will have" is not in the past. This is like:

Once I finish this pancake I will have orange juice.

Referring to making, it means after you finish cooking "this" pancake, you will have finished cooking five pancakes.