“If only we had the down payment” may be one of the most frequent complaints among aspiring home buyers in cities across the country.

Yifan Zhang, a 29-year-old entrepreneur who often hears this lament among her friends, has come up with a service that tries to help.

When she bought a townhouse in Seattle with her husband last summer, she knew that the spare bedroom could generate extra income on Airbnb. But when she learned just how much they could collect each month — enough to cover the mortgage, and sometimes more — her entrepreneurial instincts kicked in: Why not front would-be home buyers money for a down payment, and then collect a share of their Airbnb rental income in return?

That was how Loftium, a service in Seattle, came about: It will provide prospective home buyers with up to $50,000 for a down payment, as long as they are willing to continuously list an extra bedroom on Airbnb for one to three years and share most of the income with Loftium over that time.