Can I do anything to stop parishes offices being built in a church narthex?

The narthex, or vestibule, is not in the church proper. I don’t think you have much of an argument, certainly not as far as Church directives are concerned. I gave a mission in Canada, where it rains a lot. The vestibule is quite large and opens into the parish hall and kitchen, meeting rooms, the parish office and the priests’ offices. People can also just wait there until their ride arrives—all this without getting soaked. It was great!