So:
When building a house, you must move in during the year of completion and when buying, in the year of handover. This way, you do not fall into the so-called annual trap and are definitely safe for the full 8 years.
Here briefly in detail:
According to the BFH ruling of 26.7.1980, BStBl. - 1981 - II p. 152, an apartment is considered completed and therefore ready for occupancy when essential construction work is finished. This includes doors or windows being installed, connections for electricity and water supply, heating as well as sanitary facilities being present, and also a cooking facility (stove connection) being available. It is irrelevant if minor final works are still outstanding. For example, if the carpets have not yet been laid in a room of the apartment, or the walls are still not wallpapered, or if the exterior plaster has not yet been applied, or the access paths or fencing are not yet completed outside. The apartment is still considered completed for tax purposes. So if two rooms, the kitchen, the bathroom, and the toilet in an apartment can be used without restriction and only the living and dining rooms are not yet finished, moving in is considered reasonable. The apartment is thus considered completed (BFH ruling of 17.4.1987, BStBl. - 1987 - II p. 567).
The acceptance date by the so-called building supervisory authority does not matter. You also cannot argue that the apartment is not yet completed because you have not yet moved in. In doubtful cases, the tax officer tries to verify the completion date using the submitted invoices.
If you (or the relative) cannot manage to move in during the planned year of completion/acquisition, you should postpone the date of completion/acquisition. As a builder, for example, you could postpone the completion of the house by installing the bathroom only in the following calendar year. As a buyer of a house or condominium, you can specify in the purchase contract that the acquisition date falls only on January 2 of the following calendar year.
Best regards, Merry Christmas and a smooth move. Our construction starts only in March.