One of Baltimore's largest law firms, Ober Kaler, will move its headquarters by next April from the Sun Trust Bank building downtown to the 100 Light Street office tower a few blocks away.

Lexington Realty Trust, which owns the 35-story office tower, announced Thursday that Ober Kaler has signed a lease for 94,213 square feet over six floors of the tower, formerly known as the Legg Mason building.

The lease is the largest announced since Legg Mason moved last year to a new building in Harbor East, vacating 330,000 square feet on Light Street. To attract new tenants, Lexington Trust is investing more than $43 million to upgrade the building and open a parking garage across Lombard Street.

As of July 1, the 100 Light Street building is expected to be approximately 44 percent leased, according to Lexington Trust.

Ober Kaler Chairman John Wolf said the law firm considered sites in Hunt Valley, Owings Mills, Towson, Harbor East and elsewhere in Baltimore's central business district, including its present location, before leasing space at 100 Light Street.

"We believe it's an iconic building on the harbor, and we are fans of the old central business district," he said. In addition, "the economics were very favorable. ... We're very excited about this."

According to Lexington Trust and Ober Kaler, the lease expires March 31, 2026, subject to renewal options. Ober Kaler is planning to start renovations this summer and take occupancy before April 1, 2011. Gensler is the architecture firm handling renovations of space for Ober Kaler.

The lease means that 230 employees of Ober Kaler, including 98 attorneys and support staff, will move. Ober Kaler currently occupies 85,000 square feet at 120 E. Baltimore St., where it has been since 1991. Before that, it was in the former Maryland National Bank Building at 10 Light St. Founded in 1903, the law firm also has offices in Washington and Falls Church, Va.