The Bundesbank, Germany’s central bank, said recently that real estate in German cities had been overvalued by 15 to 30 percent — in other words, that there is a bubble. The UBS survey cited Munich, Frankfurt, Amsterdam and Paris as cities at risk. And a study by the global accounting firm Deloitte & Touche cautioned that average house prices “will exceed pre-crisis levels” if the European Central Bank keeps interest rates at zero, as planned.

Housing prices have risen sharply in the United States as well. But there, the boom has been driven by individual buyers, household debt has been held in check and lending standards have remained relatively tight — all factors that reduce the chance of another collapse. Moreover, while benchmark interest rates in the United States have been kept low, they were never negative — and have now been above zero for several years.

Some economists say that the concerns in Europe are overblown and that prices are overvalued but not in a danger zone. For one thing, job creation from the economic recovery, however tepid or uneven, has expanded the ranks of creditworthy borrowers. And buyers are mainly living in properties or renting them out, rather than flipping them as happened before the crisis.

The supply of urban housing, however, has failed to keep pace with the resulting demand. Disrupters like Airbnb have added to the crunch by converting residential properties into vacation stays. The result is a shortage of affordable housing, particularly in the rental sector, squeezing middle and low-income earners such as teachers, firefighters, nurses and retail employees who work in cities but cannot afford to live in them.