Is it permissible to give zakat al-mal to parents whose income is low and who are about to buy a house for which they will incur debt?

A son is not permitted to give his Zakat to his parents, because if they are poor, their maintenance is obligatory upon him. If they are rich, they are not eligible for Zakat. The fact that they live in rented accommodation and have a low income does not make it permissible to give them Zakat. Rather, it is obligatory for the rich son to pay their full rent or whatever amount they are unable to pay.