Freddie Mac is expanding its role in financing one of Wall Street’s postcrisis success stories: the booming business of investing in single-family rental houses.

The government-backed mortgage-financing firm this month guaranteed a $509 million loan that helped Front Yard Residential Corp. , an acquirer of rental homes, buy a rival. It also backed a $7.8 million loan that enabled Promise Homes Co., a midsize Atlanta landlord, to buy 117 houses in Southeastern working-class neighborhoods.

The McLean, Va., company already is the country’s largest backer of apartment loans. The deals announced this month are part of its push to finance more rental-home purchases as investors are eager to put money to work in rental markets and as growth in the firm’s traditional single-family home-mortgage business has slowed. Investors said the move could help fuel the rental boom by keeping a lid on costs at a time when U.S. interest rates are broadly expected to rise.

“Having Freddie in the business will bring down the cost of capital over time,” said Beth O’Brien, chief executive of CoreVest Finance, which lends to smaller landlords and bundles the debt into securities. Freddie guaranteed $161 million of debt sold by CoreVest last year in its initial rental deal.

Freddie Mac and its larger rival Fannie Mae don’t make loans, but buy loans from lenders and sell them to investors as securities, guaranteeing that borrowers will make their monthly principal and interest payments. Their scale and government backing typically allow them to borrow at lower rates than others.