Winstanley Enterprises is planning a 500,000-square-foot companion tower to its 100 College St. lab-office building in New Haven.

New Haven biotech firm Alexion Pharmaceuticals plans to expand its Global Product Development lab at 100 College St. in the city.

The company announced its plans Thursday, which it said will “support the continued growth and diversification” of the company’s pre-clinical drug pipeline, and will help move those drugs to the clinical study phase.

In response to queries from The Commercial Record, the company would not say how many additional square feet the expansion would occupy at 100 College St., which is owned by Winstanley Enterprises. Alexion opened its offices in the tower in 2016, but a year later announced that it would move its headquarters from New Haven to Boston, taking with it 400 corporate and research jobs. The company did, however, retain approximately 450 jobs in its New Haven office, primarily in its R&D, clinical supply and quality teams, nurse case management and enterprise business services.

Prior to Wednesday’s announcement, Alexion occupied around half of the 4-story, 513,000-square-foot building, including all of its existing laboratory space.

The company said it would spend $10 million on the expansion “and is looking forward to working with local and regional contractors to complete the project before the end of 2020.”

“This planned expansion reinforces the company’s commitment to Connecticut and to the city of New Haven, where Alexion was founded 27 years ago and has developed and maintained a strong presence,” the company said in a statement.

“When a company of this size chooses to expand in our state, it signifies strength of the relationship between our state and this growing industry,” Gov. Ned Lamont said in a statement.

Lab space in New Haven has generally been hard to come by in recent years, with rents too low to support new construction but demand steady from spin-offs from Yale and other area universities.