Is zakat obligatory on money paid to a contractor as a down payment to reserve a residence that has not yet begun construction, knowing that the annual due date for zakat has arrived?

If the precise specifications of the land or dwelling are unknown, the contract is void. The money remains your property, and you are obligated to pay zakat (definition) on it. The disbursement of this zakat can be postponed until the money is recovered. However, if the contract fulfills the conditions, it is a permissible Istisna' (manufacturing) contract, and the money exits your ownership, thus removing the obligation of zakat from you.