The Boston Athenaeum announced Thursday that it had signed a long-term lease at neighboring 14 Beacon St. that will allow the independent library to expand by almost 20,000 square feet.

“The board has long known of the need for additional space to care for our library’s valuable and ever-expanding holdings,’’ John S. Reed, the Athenaeum board of trustees president, said in a statement. “We looked at a range of options for responsible growth over time, including moving collections off-sight — a prospect soundly rejected by our members.’’

Reed said the board was pleased “to have identified a practical, cost-effective solution right next door.’’

The Athenaeum already occupies the basement and first floor at 14 Beacon. It will now add the fifth and sixth floors and is planning to consolidate its administration, IT, and some storage in the new space.

This will free up more space in their original building at 10½ Beacon St. for collections and gathering spaces. Renovation is scheduled to take place over two years; however, there is no known start date.

“The user experience will be in 10½, in the gracious interiors there,’’ Creelea Pangaro, a board vice president, said by phone. “We’ll be emptying out alcoves that are currently being used as offices and those will become readers spaces, and so it will sort of return it to a more historical aesthetic.’’

Pangaro said that members “seem to be very happy’’ with the expansion. 

“We’ve been up against space constraints for decades, and because our building is so much a part of our identity, the thought of moving off site was just never palatable,’’ Pangaro said.

But opportunities for more space occur infrequently in the Beacon Hill neighborhood. The property at 14 Beacon became available following the 2017 sale of the building by the American Congregational Association to real estate firm Faros Properties for $25.4 million. 

News of the expansion comes a week after Athenaeum director Elizabeth Barker announced her impending departure from the library, following a tumultuous tenure that has been marked by internal struggles and staff shake-ups. According to former employees, many of the disagreements revolved around Barker’s push to modernize the historic library, and a generally “hostile’’ work environment.

Lillian Brown can be reached at lillian.brown@globe.com. Follow her on Twitter @lilliangbrown.