Using N scale buildings on a Z scale layout will look pretty odd depending on the building. N is 1:160 and Z is 1:220, which is quite a difference. You'll end up with things like the front door of a house being as tall as a locomotive. Having said that, if you carefully choose your placement (off by itself and not too close to the trains or other buildings, maybe?) some buildings could work and not be too noticeable, especially if they're a building that's supposed to be small for N scale. For some structures like water towers etc. you could simply look for a smaller example in N scale and it would scale perfectly to a medium or large structure in Z. It's the doors, windows, stairs, and all that which will probably be most noticeably giant-sized.