There are a lot of "creative financing" options available these days - you may be able to get by putting only 5% down. What you'd do is take out a mortgage for 80%, and then a second mortgage for the remaining 15%. Alternatively, you can get PMI (mortgage insurance) - it's an extra payment but it goes away when either you've paid to own 20% of the home or if your home appreciates and you only owe 80% of its value left, that will also get rid of your PMI.
We were able to come up with half our down payment (10%) and my parents "gifted" the other 10% to us. At some point we will "gift" it back to them, but to get the mortgage they have to sign an affidavit saying that they don't have any expectation that we will return the money - ie it's a gift and not a loan.
Whatever you end up doing, make sure you can afford your payment. It's better to start out small and have some money to spare than to stretch and get into trouble because you become injured and unable to work or something.