Back Bay duplex in converted mansion on sale for nearly $11 million

The 4,286-square-foot, seven-room Unit 2 at 305 Commonwealth Avenue in Back Bay is part of a recent conversion of a late-19th-century mansion that the famed architecture firm Peabody and Stearns originally designed.

The spread, which includes three full bathrooms and the potential for three bedrooms, is on sale through Campion & Co. for $10.99 million.

It is the largest of 305 Commonwealth’s four units—bigger even than the project’s 4,028-square-foot penthouse, which has yet to go up for sale. There is a garden-level one-bedroom running to 551 square feet and a Unit 3 on the third floor that runs to 2,261 square feet.

Unit 2, which is on the parlor and second floors, comes with a 169-square-foot deck and two tandem garaged parking spaces.