Architect Guy Grassi of Grassi Design Group designed the interior spaces, and Martins Construction Company was the general contractor.

Not often does a developer preserve extensive hand-carved woodwork within a property during a two-year-long condominium conversion, but it’s all there in Unit 2 of the Back Bay mansion at 305 Commonwealth Ave.

“It’s unusual, and it’s beautiful,” said Tracy Campion of Campion and Company Fine Real Estate who is marketing this magnificent 4,286-square-foot duplex with three-plus bedrooms and three full baths. It also has an outdoor deck and two tandem garage parking spaces. It is listed at $10,990,000.

The prestigious Boston architectural firm of Peabody and Stearns designed and built the townhouse between 1884 and 1886 for Cornelia Patterson (Van Rensselaer) Thayer, widow of Nathaniel Thayer, a banker, railroad investor and philanthropist. She lived there until she died in 1897.

For the most part, the townhouse has had residential use, but for a 43-year period, it served as offices, beginning in 1939.

Amazingly, much of the original architectural details remained intact. When a developer, 305 Hexagon LLC, acquired the building three years ago, manager Sandra Edgerley decided to create four exquisite residences and an interior garage.

Architect Guy Grassi of Grassi Design Group designed the interior spaces, and Martins Construction Company was the general contractor. She called her project the Thayer Mansion.

Unit 2 is unique, Campion said. All of the detailed woodwork of the kitchen reflects the elaborate style of the Gilded Age, yet this space also boasts state-of-the-art amenities.

The exterior of the Thayer Mansion comprises brownstone and brick and features a curved bay that rises three stories topped by a large triangular parapet with a lion’s head.

Huge pillars and wide brownstone steps ascend to the arched recessed entry. Beyond is a magnificent interior. The vestibule has a high gilded domed ceiling and flows through a second door to an arched lobby with shoulder-high paneling and a sumptuously carved staircase.

In front of the stairs, Unit Two opens into an elegant dining room dominated by a handsome fireplace with a marble surround and an enormously wide mantelpiece with intricate carvings; the overmantel has niches for sconces on either side of a classical Greek relief.

The coffered ceiling rimmed by a complimentary frieze soars above the restored quarter-sawn oak flooring laid in a chevron pattern, and high paneled wainscoting surrounds the room.

Solid pocket doors separate the dining room from the living room, where four tall windows with leaded glass transoms look out to the Commonwealth Avenue Mall. Here is a wood-burning fireplace with a wood mantel, marble surround and mirrored overmantel. Paneled wainscoting and a deep frieze surround this room.

A huge family room/kitchen on the other side of the dining room is breathtaking. Original architectural details of the fireplace are incorporated into the kitchen. For instance, the top of the quarter-sawn oak center island as well as the backsplash, is Nero Marquina, which looks like the black marble with white swirls on the fireplace. And the custom-designed range hood features a floral motif that has been laser-cut to replicate the elaborate wood carvings of the mantelpiece.

The abundant cabinetry echoes the room's quarter-sawn oak paneling throughout the room.

High-end appliances include a black enamel Ilve Majestic dual fuel range with a pot filler, a Bosch dishwasher and a pair of Sub-Zero refrigerators with bottom freezers.

There is an ell off the kitchen that includes storage space, a den and a full bath with a shower, plus a second stairway. Halfway up the flight is access to a 169-square-foot Ipe deck.

Two bedrooms on the second level share a bath with a double vanity and glass-enclosed shower. One bedroom has a “twindle,” a built-in bed in front of the window. A hallway continues to a large laundry with a side-by-side Electrolux washer and dryer and sink and ultimately to the master suite, which occupies nearly half of this level.

Overlooking the mall, it boasts original hardwood flooring, another fireplace and a high honeycomb ceiling with a chandelier. Bookcases surround the room, once a library, and in front of the curved windows is a wide wooden seat.

A study off the bedroom with more embellished woodwork and leaded glass cabinets offers a retreat.

The suite includes a spacious dressing room lined with open shelving and hanging rods and anchored with a marble-topped dressing table.

Adjacent is a lavish master bath that sparkles with Bianco Statuario marble on the walls and floor and atop the double vanity. It includes a stand-alone 66-inch elliptical tub and a glass-enclosed steam shower.

Near the second staircase, opposite the shared bath, is an elevator that gives direct access to the garage. It has a big turntable, which allows a 180-degree rotation for easy exiting.

At the rear of the mansion is a brick courtyard that has been landscaped.