We’re building a room on to our house that also has a ceiling fan. The building inspector told us a ceiling mounted smoke detector needs to be as far away from the fan as possible. But he didn’t say whatever thing about not being able to mount it on the ceiling.

you can also mount is vertically, perhaps above his door .

Place it on the ceiling right over the door. This is also excellent for smoke that travels from outside to inside the room.

try looking at this: http://www.thehardwarecity.com/?sku=9141029

the best place would be on the ceiling without the fan running ,especially at night when sleeping .the further from the ceiling the less effective it will be possibly place one o the ceiling just outside the door as a further source.