Bulkhead in Bathroom to install ventilation fan?

Hello. We are talking to several contractors about renovating our upstairs bathroom. We live in an old semi-detached house in Toronto with a low-slope/flat roof directly above the bathroom. Right now our vent fan is attached to the wall and we want to relocate it to the ceiling to be more effective and less prominent. Two of the contractors we have spoken with have told us we need to build a bulkhead in the bathroom to locate the fan and vent directly out the side of the house. I understand there are concerns about venting through a flat roof but I also follow real estate listings pretty closely and I have never seen a bulkhead like this in a similar bathroom. Is this there a reason it couldn't be installed in the ceiling, run the ductwork between the joists, and down through the wall to vent outside? I assumed this was how many bathrooms in our area were vented so am surprised to have heard this twice now. We are concerned about having an ugly bulkhead in our bathroom since we might be selling our house in a few years.