The same: "You are building a nice house."

English, at least in standard formal varieties, doesn't distinguish between singular "you"(wewe / u- / -ku- / -ako) and plural "you" (n(y)inyi / m(w)- / -wa--(e)ni / -enu). They're the same, so in sentences like this, they need to accept mnajenga and unajenga because we have no way of knowing which answer they want from the English.