The superrich have been busy scooping up luxury pads around the world. Last year, they liked nowhere better than the U.S.New data from a study of “ultra-high-net-worth individuals” – richer still than the 0.1% – show that their swelling ranks are increasingly storing wealth in multimillion-dollar mansions around the world.

When a location gains favor with these cash-abundant buyers, defined as those with at least $30 million in assets, luxury home prices tend to jump. And in 2014, New York and Aspen saw prices rise 18.8% and 16%, respectively, putting the cities first and second on a list the 100 global locations featured in Knight Frank’s Prime International Residential Index. (You can see the full index here.)

Renewed confidence in the U.S. economy has helped attract wealthy investors “focused on key safe-havens, where they consider the market safe enough to put their money into bricks-and-mortar,” said Kate Everett-Allen, head of international research at the firm.

San Francisco and Los Angeles were also in the top 10. Overall, luxury property in U.S. cities analyzed saw prices rise by almost 13% on average.

Others regions were shunned, especially second-home destinations in Europe like Cannes and Tuscany. Compared to the U.S., “the disparity with Europe’s cities is stark,” said Ms. Everett-Allen said.

Prime-property prices in Europe fell 0.4% in 2014. Strong performances in cities like Dublin, Berlin and Amsterdam couldn’t overcome falling prices in 31 European hotspots.

Globally, prime property prices rose by just over 2% in 2014. This was less than the 2.8% rise in 2013, but the broad trend remained on track: The rich are getting richer, and they like splashing out on fancy homes.

This “relatively simple narrative, undented by even the global financial crisis, has dominated our analysis” since the report was first published in 2007, said Liam Bailey, global head of research at Knight Frank.

The number of ultra-high-net-worth individuals rose 6% to 211,275 in 2014, according to a report last Thursday from Wealth-X and Sotheby’s International Realty.

Cumulatively, these UHNWIs are worth $29.7 trillion, and of that, $2.9 trillion is held in residential real estate, with each UHNWI owning 2.7 properties on average, the Wealth-X report says.

All this cash has been, and is set to remain, good news for high-end estate agents and property developers: the Knight Frank report said over a quarter of UHNWIs are considering purchasing another home in 2015.

However, the taxman has noticed. “There is increasing government scrutiny of wealth and levels of protectionism,” the report said.

Policies aimed at cooling the Dubai housing market, such as tighter mortgage lending criteria, pushed luxury prices down 0.3% in 2014, from 17% price growth in 2013. Hong Kong and Singapore have also saw prices drop last year.

In New York, Paris, and the U.K., new policies are on the table to tax high-end or second homes. For now, “those lucky enough to have property in the U.S. are unlikely to have any complaints, as domestic and international demand fueled price growth,” the report said.