For the most part, you aren't allowed to borrow to come up with your down payment. However, there is an exception. If the loan is secured against some asset, you can borrow the funds.
For example, if you take out an equity line on your present house, you can use those funds to make the down payment on your next home. A lot of people do this when they intend to rent out their previous home. It also works in case you aren't certain of the housing market. Since equity lines are very inexpensive, it is a simple process to line one up before you put your own house on the market and begin looking for a new home. That would allow you to make a "non-contingent" offer, giving you more viability as a potential buyer.
