A mortgage specialist said today that U.S. home prices will probably decline an additional 6 percent to 8 percent before bottoming, Bloomberg reported.

Potential home buyers are being kept on the sidelines by policy-makers tightening rules for government-backed loans and banks being more restrictive than required by Fannie Mae, Freddie Mac and the Federal Housing Administration, Scott Simon of Pacific Investment Management Co. said in a Bloomberg radio interview.

“If you can borrow, housing is so cheap, but if you can’t borrow, it’s infinitely expensive,” Simon said.

The worst housing slump since the Great Depression has so far driven property values down by about 31 percent since a mid-2006 peak, according to an S&P/Case-Shiller index.

Drops of 15 percent to 20 percent more would become a “dire situation” because that would leave nearly half of U.S. homeowners with mortgage balances that are greater than than their properties’ values, up from about a third now, Simon said.