Joel and Kathryn Friedman, both 71, are counting the days until they can sell their home and move into a 55-plus community.

The retired empty-nesters have been ready to downsize for years, but are reluctant to sell their five-bedroom, 5,000-square-foot Southern California house [mansion] in large part because of at least $700,000 in capital gains taxes they estimate they'd have to pay.

Since 1997, home sale profits over $500,000 (for married couples) and $250,000 (for single filers) have been subject to a capital gains tax of up to 20%. That threshold hasn't changed since 1997, meaning that — between inflation and soaring home prices pushing an ever higher number of houses above that limit — many more home sellers have to pay the tax now than when it was first implemented.

The Friedmans are among a growing number of older homeowners discouraged by the tax from selling their valuable properties. Housing economists say that dynamic has exacerbated a shortage of family-sized homes on the market, especially in expensive places like California.

The Friedmans' house is too big for them, and maintenance costs are only rising, Joel said. "There are a million reasons why we'd like to move, but we're not because the tax is just burdensome," he said.

But that could change — there's bipartisan support in Congress for raising the federal tax threshold to boost home sales in a stagnant market.