Country Grammer

So I'm reading about the exploding population in northern Tarrant County and come across this sentence: A significant number of houses is also being built in southwest Fort Worth, as well as in the central city, including in and around downtown. I feel like an idiot. It seems like that should be "are" but I think it has something to do with a prepositional phrase. But I have no idea. I'm spending the rest of the night diagramming sentences while smoking marihoochie and playing bongo drums.