Interim financing.
To get started, my wife and I took out a second mortgage on our own home. That provided the capital for the first house. Unless you are an established customer with a bank, perhaps as a builder, very few bankers will provide interim money for building a house for low-income families, from trash.   Typically, bankers simply don't recognize that as a standard profit venue. New ideas must deal with the present reality of banking. But that is how we started. There are other ways. Acquire a plot of land--banks will loan money on land. Then on the weekends build the house. It may take awhile, but know that you are capitalizing your eventual after-market house business.

Financing for the buyer.
If the families that I build for have any credit at all, it's bad credit. So I don't check their credit. I check their commitment to their family. Very few families don't feel the fever of owning a home. The credit risk is mitigated by the fever they have to own a home, especially if their mortgage, taxes and insurance equal what they are now paying in rent, with no down payment.
Early in the process, I ask a family if they would be interested in buying a house, provided there is no down payment, or closing cost and the monthly payment would be equal to what they are now paying in rent. Who would say no? Generally, the worst credit histories can be brought back to a healthy standing with three years' good credit record. When I carry the note, I charge 10% interest, since I am not a bank. I handle it as a lease, with an option to buy at the appraised value of the house when they first move in, and the amounts paid toward the principle before they exercise the option are applied to the price of the house when they do exercise the option. It is pretty much like renting for the first three years, with a partial refund when they exercise the option to buy. The family must exercise its option to buy within three years' time. Bankers tell me that if a family makes timely payments on the lease, keeps utility payments in good standing, and makes a disciplined effort to keep financially stable, then most families would be able to qualify for institutional financing after three years. By handling the note in this fashion, title to the property is not transferred until they exercise the option to buy. This means the property can be used as collateral with a bank for interim financing for the next project. Further, after three years' time, a down payment becomes less of an issue, because they have been paying toward the principle over three years, which becomes part of a down payment that a bank would require. Additionally, the property may have appreciated slightly over three years. These two things make in-roads into a down payment. If not, the balance of the down payment can be handled as a second lien. This strategy also defers buyer's closing costs until the family is in a stronger position to manage them. An escrow account could be set up, deposits to which are included in their monthly payment. If the family drops the ball on the agreement, the "lease with an option to buy" is void, and it simply is as though they are renting the building. Clemency on this point is something the builder will have to decide. But if it is explained to the family what the stakes are, that they have the possibility of a fresh beginning, they make an effort to change. The family has a whopping opportunity, and you have protection of your investment.