This is what you call bridge financing to allow for the purchase of the new house. Unless your old house is selling or closing at a specific date, they will not fund the new house. In the meantime you would have 2 mortgages, 1 for the new property - with interest only payments (bridge) and one for the old - full P I. It is always prudent to sell first before buying as this creates additional cost. You can also rent one of the properties if you plan to keep it longer.