Commonwealth Avenue duplex with 11 rooms, seven fireplaces asks $10.5 million

The 11-room, 5,930-square-foot Unit 3 at 257 Commonwealth Avenue in Back Bay is on two floors of the old Cochrane Mansion that McKim Mead & White designed (think the Boston Public Library on Copley Square or Symphony Hall, among many other famous creations).

Because of that, the spread includes that 19th-century pile’s old entertaining rooms, which explains some of its sweep.

It’s otherwise a colorful affair, with bright walls and moldings. The unit includes three full baths and the potential for four bedrooms as well as an elliptical staircase and a library and a living room with bay windows. The tag covers two garaged parking spaces too.

How much is that tag? Given the location and the size, it’s predictably steep: $10.5 million through Marsh Properties.