The truth of the matter is that you just don't use бы besides a future tense verb to form conditionals.
Если я сниму квартиру, договор будет бы на 11 месяцев?
That's what I am talking about and this бы after a future tense verb is probably what's giving you that feeling, that something is wrong. As for a conditional clause mixed with future tense in one sentence. I guess it's kind of hit and miss. Like I think something like "я бы пошёл, если ты захочешь" sounds quite natural but "если бы я снял квартиру, договор будет на 11 месяцев" sounds a bit off, like the two parts are sort of logically disconnected. it isn't the greatest of errors though and sometimes you can hear such disjointed sentences, even from natives.