Someone has brought up this topic before and I think the biggest con was insurance carriers consider a coop attached to to a home as a fire hazard, zoning ordinances might prevent it too. In some cases a coop has to be a certain number of feet from any house. You might want to check into this before building.
Welcome! You've responded to somebody who hasn't posted anything in over 12.5 years. You may be better off starting a new thread to get a new set of eyes to your questions.