The building application does not seem to me to be a relevant point in time at all. It only mentions the issuance of the building permit. And I don't find the text there clear at all. The first child must be born at the time of issuance of the building permit or purchase contract, otherwise it is not a "purchase as a family" (does not qualify but apparently keeps the option open for a future construction project), but otherwise nothing is stated there. Just as little as for other special cases. Of course, it would be nice if the problem could simply be circumvented by submitting the application later. Overall, however, it seems unpolished to me.