MIAMI—When you think of Miami's hidden treasures, you probably think of South Beach's Art Deco or North Beach's MiMo architecture. But churches are on the radar screen of many developers in today's market.

That's because developers that want to build taller and denser projects need air rights to take their projects from the drawing board to the clouds. Churches and other historic sites around Miami are gold mines if they possess unused air rights.

According to Javier Avino, a partner in Bilzin Sumberg's Land Develop & Government Relations Group, the market for air rights has taken off since the implementation of Miami 21 zoning code. The code allows owners of historic buildings to sell to developers in areas like Downtown Miami their unused air rights so long as they reinvest the proceeds in renovations. In fact, this robust market is helping to revitalize neighborhoods like MiMo where residents are selling their air rights to renovate run-down motels, restaurants and stores in the neighborhood.