We're finishing our walk-out basement and have a bathroom at one end of the house (underground part). We need to vent the exhaust fan to the other end (walk-out part). The bathroom was roughed-in by the builder so they put it in that area.

The problem is that my contractor says the length of the vent is too long. He thought he remembered 25 feet is the maximum length for a bathroom bent and you have to take off 2.5 feet for each 90 degree bend. The length from the bathroom to outside is about 28 feet and we will have one 90 degree bend. I asked him to show me the code but he hasn't gotten back to me.

The builder put the bathroom there so I'm assuming they knew the codes. Going through to the roof would be about the same length. BTW it's a 4 inch vent. Any ideas?