Clayton-based developer Green Street has acquired HDA Architects, a major St. Louis architecture firm. The terms of the deal have not been disclosed, but HDA will continue to operate under its existing name and retain all 16 employees while working within Green Street Building Group, the firm’s multifamily and hospitality division. HDA will also continue to undertake projects independently of Green Street. While the two companies will initially remain in their separate offices, there are plans for a future combined headquarters. The acquisition is part of Green Street’s strategy to expand its national pipeline, leveraging HDA’s extensive experience and national contacts.