Is zakat obligatory on money saved for the purpose of purchasing an apartment under construction, of which a down payment has been made as a reservation?

If money reaches the nisab (minimum threshold) and a hawl (full lunar year) passes over it, zakat becomes obligatory on it. The fact that it is saved to buy an apartment does not prevent the obligation of zakat. However, if the actual contract has been concluded and the remaining amount has become a debt, then the preponderant opinion is that zakat is obligatory on this money if you possess assets that can be used to offset the debt, and these assets exceed your needs and the needs of those whose maintenance is incumbent upon you; otherwise, zakat is not obligatory.