Large, high-end homes across the Sunbelt are sitting on the market, enduring deep price cuts to sell.

That is a far different picture than 15 years ago, when retirees were rushing to build elaborate, five or six-bedroom houses in warm climates, fueled in part by the easy credit of the real estate boom. Many baby boomers poured millions into these spacious homes, planning to live out their golden years in houses with all the bells and whistles.

Now,...