Boston’s housing chief expects “thousands” of apartments to suddenly hit the market in December when the short-term-rental regulations fully go into effect, forbidding people from listing investment properties on Airbnb.

“We’re hoping that the units returning to the market will be in the thousands,” Boston housing Chief Sheila Dillon told the Herald, though she said it’s still unclear quite how many apartments that will mean.

She said an earlier estimate from Airbnb — which isn’t the only short-term rental company, but is the most popular — said about 3,000 units in Boston have been in the website for most of the year, suggesting the owners don’t live there. That would be in line with the trend up to this point, as people have attempted to register about 1,000 of the 6,100 Airbnb listings — and half of those applications have been rejected. Come Dec. 1, people who aren’t registered with the city won’t be able to list their units on the rental site.

“We’re hoping that more units coming on the market will increase the vacancy rate and units will continue to moderate,” Dillon said. “If we’re building additional housing to influence the rental cost, we need the units that were built to be residential units built to serve full-time residents.”

Airbnb has declined to say how many units it is expecting to boot out.

Dillon, speaking at a council hearing Tuesday, also noted that this could drive down the number of houses being intentionally kept empty and unused, which cause some neighbor complaints.

At the hearing, Jerome Smith, the mayor’s civic engagement chief, pointed to some luxury buildings, like new ones in the South End, where it appears short-term rental companies have scooped many units up.

“We hear from residents in the building where they don’t even know if they have another resident on the floor,” Smith said. “This whole apartment-sharing industry is finding new ways to get this in there … We need to put our hats on because the residents are asking this from us, and we haven’t found a sound strategy about how we actually know what’s open.”