WASHINGTON — Developers PN Hoffman and Madison Marquette haven’t even started work on their big Phase 2 addition to The Wharf along D.C.’s Southwest waterfront, and they have already signed a major tenant.

Law firm Williams & Connolly LLP has signed a 15-year lease for 300,000 square feet in a pair of Phase 2 office towers as the anchor tenant and will relocate from its Metro Center location — where it has been since 1992 — when Phase 2 is finished in 2022.

Williams & Connolly, with more than 600 attorneys and staff, will occupy the majority of the 500,000-square-foot towers at 690 Main Ave. SW.

“From our founding more than 50 years ago, this firm has been regarded as a Washington institution and we have always been proud to be a part of the fabric of the Washington, D.C. community,” said Williams & Connolly Chairman Joseph Petrosinelli.

“As we look to the future, we are excited to be at the forefront of the further development of the city taking place along the Waterfront. It is the perfect location for Williams & Connolly to launch its next 50 years.”

Williams & Connolly’s lease includes a penthouse level.

JLL represented Hoffman-Madison Waterfront in the lease. Savills Studley represented Williams & Connolly.

The Wharf’s Phase 1 opened in October with 2 million square feet of residences, offices, hotels and retail with marinas and waterfront parks. When complete, the $2.5 billion, mile-long Wharf will include more than 3.5 million square feet of development.

Phase 2 construction will begin in early 2019.