(FORTUNE Magazine) – For hot housing markets of the future, look to retirees and vacationers. "Within a decade the first edge of the baby-boomers will be looking at retirement, at age 55, not 65, and they are likely to be more mobile than their parents," says Kenneth Rosen, a real estate expert at the University of California at Berkeley. "Normally only 5% to 6% of retirees move, but that will shift up, to 15% to 20%. So Sunbelt locations will be stronger than the Frostbelt." If that seems an ambitious forecast, consider that in 1960 when Del Webb Corp. opened its first Sun City retirement community in Arizona, only 2% to 3% of retirees indicated a willingness to relocate. Retirees are changing in other important ways that will affect demand for housing. They're healthier and wealthier and want to live where they can swim or golf or play tennis. "We don't build shuffle board courts any longer," says Ken Plonski, Del Webb's public relations director. At the same time that more retirees are migrating far from their homes, many others want to remain near family and old friends. Retirees from MinneapolisSt. Paul are buying lakeside properties in Amery, Wisconsin, just fifty miles away. Del Webb recently opened its first community outside the Sunbelt, in Sacramento, to tap into the local market. Combined with the retirees, growing demand from baby-boomers -- now at the age when people buy second homes -- is pushing up values in many vacation spots. Century 21 Real Estate Corp. says prices in 21 strong markets rose 18% in 1993. In Amery, realtor David Hicks of Century 21 Premier Group cites a local home that sold last year for $87,000, resold in early spring for $110,000, and sold again just recently for $120,000. St. George Island, Florida, broker Alice Collins says some beachfront lots have surged to $320,000 from $150,000 two years ago. "We are getting a lot of interest from retirees and from professional couples -- usually in their early 40s to 50s -- who are buying for future retirement," says Collins. Anyone with a sense of history and demographics can see the reasoning behind that strategy -- home prices for primary residences surged when baby-boomers crushed into the market in the 1970s. What's to say it won't happen again when this group hits the retirement-home market? Other developing trends that could boost vacation-home demand and prices: --Telecommuting. Cape Cod towns are getting upgraded telecommunications systems to accommodate New Yorkers and others who want to spend Mondays or Fridays at their home offices on the island, says realtor Jamie Regan. --Foreign buying. "It's cheaper for the Germans and British to buy here than in Spain," says Marie Powell, who runs a brokerage in TampaSt. Petersburg. --Shortages of choice sites. Available waterfront property is dwindling in the Tampa area. On Cape Cod, environmental restrictions limit construction on undeveloped lots. To get around such constraints, some people are buying existing houses on the water, tearing them down, and starting afresh, which is allowable under existing environmental laws.