Taking back full control of the company, architecture firm Foster + Partners has purchased a minority stake of the company from private equity firm 3i Group plc.

Foster + Partners is headquartered in London and boasts offices around the world, including New York. Previously, 80 of the firm's partners had a stake in the business, but now with the acquisition, 140 partners will have ownership.

The firm’s designs include the following buildings: Apple Inc.'s (Nasdaq: APPL) U.S. headquarters; airports in Jordan, Kuwait, Qatar and Panama; Bloomberg's London headquarters; 425 Park Avenue, the first new building on Park Avenue in 50 years; and the Global Financial Centre in Shanghai's Bund district.

The 3i investment allowed the company to diversify geography and services, as well as transition to a partnership model. Since 3i's investment, Foster + Partners has almost doubled sales in the U.S., which now accounts for 27 percent of the practice's billings. The firm also grew in Asia, which had 8 percent of its billings in 2007, and now also has about 27 percent.

London-based 3i received cash proceeds of £70 million (about $119.2 million), which combined with loan repayments to date brings the firm's proceeds to about $306.5 million. The firm also will receive a $68 million in deferred consideration, giving it a 1.8 times multiple on its investment, according to the company. 3i invested in the company in 2007