My situation is a bit complicated. I live in a five unit building, one being the basement. There have been rats in the basement for almost a year now that the caretaker has been aware of it. He has not done anything to take care of the problem, so the rats have now moved up from the basement into my apartment (first floor). I called the caretaker twice to inform him of the problem and received no answer. I have a newborn and am concerned for her health/safety and thus have removed myself from my apartment for about a week. I also took it upon myself to have the bottom two apartments and the basement inspected by a reputable pest control company. I paid for the estimate and they told me the initial treatment would be $600 plus $80 a month to ensure extermination. They also said it's imperative that we clean out the debris/furniture in the basement so the treatment would be effective. I have someone lined up to clean out and haul away everything in the basement for a flat fee of $200. Since I couldn't get ahold of the caretaker, I tracked down and called the owner. The owner has responded and agreed to everything, suggesting that I pay for everything and deduct the costs from my rent ($1000/month). I also have a problem with a broken dishwasher that fills up and floods onto my floor that the caretaker has been aware of and there is a new problem with HUGE flies in two units that I don't think he knows about.
I am already one month behind on rent, and have not payed this months rent either. So really I was just behind a month, not withholding. Now that I haven't payed this month I'm "behind" two months. Can I withhold rent further? Should I pay for the treatment and count it towards rent? What are my caretaker's legal obligations in this situation? What are my rights? I have the agreements of the owner to count the treatment as rent in writing.
Thanks

Withholding rent
Nobody should have to live with rats in their home. However, I don't think the type of arrangement you suggest is a good idea. In general, trying to withhold rent when you are already behind on the rent is not a good plan, and not supported by MA law. I would force the landlord to take care of the rats. You do that the same way you would force the landlord to make repairs. If you do pay for the exterminator out of your own pocket, make sure you get something in writing from the landlord before you do it, then get receipts for all money you spend trying to fix the problem.