Property laws in Massachusetts are unique so I think that it may be best for you to consult with a real estate attorney in your area.  My gut instinct is to say no, if the rent included utilities it includes utilities.  While the addendum is a nice idea it does not sound like it will stand up in court.  I would advise to tell the tenants no, that the rent does not include utilitites and that they must pay their own. It may be in your best interest to lower the rent a few bucks in good faith to show them that you want them as tenants (assuming that you have checked them out and that you do want them).  Nothing big but a good faith gesture.  Better to lower the rent than lose the tenant sometimes.  Good luck.