The pipeline for new apartments in old office buildings is filling up fast.

Why it matters: Converting unloved, underused office space is a key part of many big cities' plans to adapt to the post-COVID era. It helps solve two problems at once — putting empty space to use and creating much-needed housing.

State of play: Over 55,000 new apartments are expected from conversions right now, quadruple what it was in 2021, according to RentCafe's latest annual report on the topic, out Monday.

• "Over the last four years, we've seen extraordinary growth in the conversion of office buildings into living spaces," the report authors write.

Go deeper: 46 NYC office buildings could convert to apartments under city's new plan