Language evolves, and this is a great example. From the Oxford English dictionary: (note the last sentence)
3.b. In pl. In extended use. A house or building together with its grounds, outhouses, etc., esp. a building or part of a building that houses a business. Now also occas. with sing. concord.
3.b. In pl. In extended use. A house or building together with its grounds, outhouses, etc., esp. a building or part of a building that houses a business. Now also occas. with sing. concord.