Officials at Massachusetts General Hospital, one of the largest and most-prominent medical centers in the Boston area, are proposing a major addition that would add 1 million square feet and hundreds of single-bed patient rooms to the hospital's downtown campus.

In a letter to staff Tuesday, Mass. General's president, Dr. Peter Slavin, and CEO, Dr. Timothy Ferris, say the expansion will help the medical center keep up with growing demands for its services and changing medical technologies.

They anticipate that the new facility — designed to include a pair of 12-story towers with underground parking — will house a cancer center and a heart center, along with space for a variety of inpatient and outpatient services, and some 450 single-patient rooms.

"Single-bed rooms — the national standard of care for new hospital construction — offer privacy, space for families, and a quiet and calm environment to foster recovery and healing," Slavin and Ferris say in the letter.

According to MGH, only 38 percent of its beds are currently in single rooms, which it says lags "well behind national peer institutions."

The cost of the expansion is estimated at more than $1 billion, and the hospital says donors will cover 20 to 30 percent of that cost. Slavin says Mass. General will remain focused on controlling the cost of its care.

"We as an institution have come in below the state benchmark since those benchmarks were established," Slavin tells WBUR. "We're committed to doing it for the long term, so we're not going to let this building drive up the cost of health care."

Mass. General is one of several area hospitals considered "high cost" by the insurance company Blue Cross and Blue Shield of Massachusetts. It is also the largest hospital in Massachusetts, with more than 1,000 operating beds, according to an analysis by the Boston Business Journal.

The new building would be located within the footprint of the hospital's current campus, with a facade along Cambridge Street. A second, smaller building is also proposed on Blossom Street.

The planned expansion at Mass. General requires approvals from state and local regulators. Hospital officials say they hope to begin construction in 2020 and complete the project in 2026.