In most of Europe, rental-apartment ownership is fragmented. That is beginning to change. In the U.K., France and Spain, institutions are accumulating multifamily properties. The fledgling industry in Europe is being powered by demographic changes, strong real-estate sales, opportunities created by the financial downturn and government actions designed to encourage development. One sign of growth: foreign investment. Legal changes also are paving the way. Meanwhile, the political winds are also changing when it comes to the multifamily sector.