Tiny Palm Beach Shores on Singer Island works hard to maintain its "Old Florida" feel, limiting building heights on the water.

PALM BEACH GARDENS — Over 500 new luxury condominiums are coming to the waterfronts of northern Palm Beach County. In the past year, Florida became a second or new home to droves of Northerners.

Dozens of luxury beachfront condos and hotels in Surfside, Bal Harbour, Miami Beach and Sunny Isles are sinking into the ground at rates that were “unexpected,” with nearly 70 percent of the buildings ...