Are the heirs liable for the deceased’s debts and, if yes, under which conditions?

If the heir has unconditionally accepted the inheritance, he or she is fully liable for the debts of the deceased (Article 4: 182 BW).
If the inheritance is accepted subject to inventory, the heir is liable for the debts only in so far as these are covered by the entitlement from the estate. He or she is not personally liable.