According to Zillow, proposed reforms to the government-sponsored enterprises (GSEs) that guarantee the majority of US home loans, could drive up monthly housing costs and diminish housing affordability for many Americans.

Congress is considering changes to Fannie Mae and Freddie Mac to reduce the risk to taxpayers if the housing market crashes again. The GSEs, which guarantee a majority of all home loans against defaults, have been under government conservatorship since 2008, when they required more than $150bn in taxpayer funds as a result of foreclosures during the housing crisis.

But Zillow analysis shows that potential changes would cost borrowers as much as $400 a month in mortgage costs. The guarantee from Fannie and Freddie is thought to keep interest rates for 30-year fixed-rate mortgages low, and housing relatively affordable. If that guarantee is changed, the typical American borrower could be facing shorter loan durations and/or higher rates.

“Some GSE reform proposals could lead to the end of the 30-year mortgage as we know it, which has long been the bedrock for financing homeownership in America,” said Zillow senior economist Aaron Terrazas.