Rooftop terrace in Hessen without a permit? Who knows?

It is not stipulated when the construction project must be completely finished. If the roof terrace is approved, you can also build it only in a few years. If you do not include it in the building application now, you would have to have building documents prepared again later for notification, which would not have to be approved (provided the legal situation does not change) but would require an authorized person for building documents.

I somehow assumed 3 years from approval.

I agree.

That sounds good so far. The question is of course whether there would be another reason to include the roof terrace when determining the building class. Regarding the requirement that a second escape route might be necessary, I found on the internet the statement that roof terraces are not considered a living room but are definitely regarded as an area for staying. A term that I have not otherwise found in the HBO, the land use ordinance, or the building code. Could this still have an impact on the building class?

I somehow assumed three years from the approval.

If nothing at all happens, you are right. §64 HBO (7) 1Building permission expires if construction work has not begun within three years of its issuance or if construction is interrupted for one year. Who wants to prove that nothing happened for a year? If necessary, you can lay a few more terrace slabs on the roof every year.

Could this nevertheless have an impact on the building class?

No idea.