Atlanta law firm Smith, Gambrell & Russell LLP is relocating its headquarters to a new 31-story Midtown office tower within the $400 million mixed-use project 1105 West Peachtree.

The firm, which has about 120 attorneys in Atlanta, will move from 1230 Peachtree St., also known as the Promenade building. It’s been in the building since 1991. It will relocate in 2021.

Smith, Gambrell & Russell signed a long-term lease to occupy the ninth through 12th floors of the new tower, which will overlook the well-traveled West Peachtree corridor just south of 14th. It’s where developers have recently added several residential buildings.

Selig Enterprises is developing 1105 West Peachtree. It started site work and will begin to go vertical with the new building next year. Smith, Gambrell & Russell is the first tenant to sign a lease in the office tower.

The firm, which has large practices in aviation, real estate, intellectual property and corporate litigation, is one of the city’s oldest — with roots dating back to 1893. It is the city’s 12th largest law firm based on the number of attorneys in Atlanta, according to Atlanta Business Chronicle’s Book of Lists, with total Atlanta staff of about 200.

Nationally, it has continued to expand. Last month, it entered Miami. Last year, it opened a new office in Los Angeles. Globally, it has expanded in the past year with new offices in London, Southampton and Munich. It also carried out a larger expansion of its New York office.Smith Gambrell & Russell now has 250 attorneys in 10 offices globally.

Stephen M. Forte, the firm’s chairman and managing partner, called the relocation of the headquarters to 1105 West Peachtree transformative. He said the firm was drawn to the new tower’s modern architecture and extensive amenities. Both features fit the firm’s evolving culture, Forte said.

Smith, Gambrell & Russell will lease 102,320 square feet in the tower, with direct access to the Sky Plaza — an acre of amenities on top of the project’s parking deck including a pool, green space, bar and restaurant.

The 3.6-acre project will also have a 178-room Marriott Autograph Collection hotel and 64 luxury residential units. The entire development is being designed by the architect Rule Joy Trammell Rubio.

The law firm’s decision to move means it can custom design its office space, said Jay Schwartz, partner and head of the relocation committee. That will help attract and retain talent as the firm continues to grow, he said.

While many law firms in cities such as London, New York and San Francisco have offset soaring rents by opting for open office designs or reducing space, those trends haven’t filtered down to Atlanta’s legal industry yet, at least not broadly, said Steven Swicegood, managing director, principal, with the architect Gensler.

At the top of the Midtown office market, rents are around $50 a square foot for new space.

“Many firms have largely chosen to maintain the status quo when it comes to design, except that they are putting more social hubs in these new offices,” Swicegood said.

“They want a place where people can connect.”

For months, Selig Enterprises has reported steady inquiries from office tenants about the 1105 West Peachtree project. Other leases may be on the short-term horizon.

It is the first project in Midtown since the company launched its own development arm, said Steve Selig, CEO of Selig Enterprises. He described Smith, Gambrell & Russell as one of Atlanta’s most distinguished and respected law firms.

Eric Mandus, partner in the Atlanta Real Estate Practice, participated in lease negotiations. Shea Meddin, Andrew Ghertner, Carla Williams, and John Izard of Cushman & Wakefield represented the law firm.

Midtown has several new office projects underway along West Peachtree. For example, MetLife Inc. received conditional approval earlier this fall to move ahead with its 9-acre mixed-use development at West Peachtree, 17th and Spring. Cousins Properties Inc. has a site at West Peachtree Street a block from Technology Square.

The 30-story tower would rise on slightly less than an acre between 7th and 8th streets.However, Selig Enterprises appeared to be the furthest along, Cushman & Wakefield’s Ghertner said.

“In our assessment, they were the most prepared to get started,” he said.

Chris Ahrenkiel and Steve Baile of Selig Enterprises represented the developer.

The new 1105 West Peachtree project has ushered in a new era for the nearly 100-year-old company, which was largely known for investments over much of its history, not development.

However, spurred by the late Scott Selig, the company launched a new development division and positioned itself to become one of the city’s most active developers of new mixed-use projects over the next two decades.