Can I stop parish offices being built in the church narthex?
The short answer
Probably not on the basis of Church directives. The narthex, or vestibule, is not part of the church proper, and such arrangements can work very well.
The narthex, or vestibule, is not part of the church proper, so there is little argument to be made, certainly not from Church directives.
Such a layout can work well. One rainy Canadian parish has a large vestibule opening onto the parish hall, kitchen, meeting rooms, parish office and priests' offices, where people can also wait for rides without getting soaked.