The D.C. Bar will pay an estimated $70 million to build a 100,000-square-foot building in downtown Washington to serve at its new home.

Owning a building, rather than renting an office, is expected to save the bar at least $25 million over the next 30 years, according to bar officials, who announced the move on Tuesday. They expect to finish construction by early 2018.

“This was the best opportunity that we've been presented in 43 years for a long-term cost control,” said bar president Timothy Webster. Webster, a partner at Sidley Austin, said the bar estimates the project will cost between $67.5 to $70.5 million.

The D.C. Bar, which has more than 100,000 members, is one of the four largest unified bars in the country. In the other three states—California, Florida and Texas—the bars own their buildings. More than half of medium- to large-sized state bars own their buildings, according to American Bar Association statistics quoted by the bar.

The D.C. Bar will increase its footprint in the new space. The bar pays more than $3.6 million annually to lease 52,375 square feet at 12th and K Streets in northwest Washington. At the new location, less than a mile east at 4th and I Streets, the bar will have 68,000 square feet. The rest of the building will be leased to commercial or retail tenants, with income going to the bar.

Webster said bar officials looked at nearly 40 sites in Washington, including new office buildings, buildings that needed rehabilitation and vacant lots. The 4th and I location was cost-effective and centrally located, he said. The land will cost $20.5 million, according to the bar.

Kenneth Schwartz, a partner at Arnold & Porter, is the lead attorney for the D.C. Bar on the project. The bar is also working with Allison Prince of Goulston Storrs on land use and zoning issues.

The bar bought the land for its new building from Quadrangle Development Corporation and The Wilkes Company, which is pursuing a 2.1 million development in the Mount Vernon Triangle area. Bar officials expect to close the land deal and begin construction in spring 2016.

The bar will join Arnold & Porter and Venable in moving east to new buildings around Mount Vernon Triangle, one of the many hotbeds of new development in D.C. Webster said the presence of other firms was “reassurance” to bar officials that they picked a site that would be convenient for members.

Webster said he hoped the bar “will be a positive force in attracting other law firms and lawyers to move down into that area over time.”

Bar officials don't expect construction to substantially affect bar dues. At most, they said that dues would go up by $7 over the next five years. Once the building is complete, bar officials said they hope the stability of owning space will ease pressure to increase dues even if other operating costs go up.

An earlier version of this story misspelled Allison Prince's name.