LONDON—Nicole Vloeimans, a 31-year-old worker at an NGO, lives in the very center of this expensive city. She has a 755-square-foot floor to herself. She pays £389 a month, about $635.

"I don't really know what to do with the space," says Ms. Vloeimans. "I don't have that much furniture."

There are some hitches. No parties...