If none of the above your leverage/rights might be limited but certainly worth a call to your attorney for some guidance.
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The best way to handle these situations is to have the closing agent set aside a specific amount of money to be used for the expressed purpose of repairing the problem. Any amount unused would be returned to the seller once the problem was corrected.
As you can see, to assume someone is going to take care of their responsibility without a more serious commitment can leave you holding the bag. Your legal options would be best addressed by an attorney but after the fact issues can be difficult at best to resolve.
That said have your attorney contact the sellers attorney and ask that it be correct or compensate you for the repair and that they should be 100% responsible. (you have a p&s that states (or should state) that the home will be delievered in the same condition as that at the time of inspection. And you and your agent notified them prior to closing)
If that does not work, either you or your attorney (may cost more than your repair) send a demand letter (go to small claims in Dedham to get the forms) This will give them 30 days to answer the claim, if they do not respond, under the demand you may be entitled upto 3 times the cost of the repair under ch 93 as it is a consumer issue,
Good luck. Keep us posted and yes as a resident of Needham since 1985 it is a great place to live and raise your family.
Second, it seems to me that your attorney and also your agent dropped the ball on this. At this point, you've closed on the house and you likely have no negotiating leverage what so ever to get the seller to pay for this repair. You may be able to take the seller to small claims court if you can prove that they somehow violated the terms of your contract.
Really, the appropriate way to handle this would have been to arrange to have some of the seller's proceeds held in escrow to cover the possible cost of the repair. In this scenario, if the seller repairs the problem to your satisfaction they get the remainder of their proceeds. If they don't, then you'll get those funds which will allow you to make the repair. Typically, just having the money held aside is sufficient to motivate the seller to make the repair. Alternatively, you might have been able to negotiated a credit from the seller against your closing costs equal to the amount you estimate for the repair. This would put cash in your pocket which would allow you to make the repair.
The time to negotiate these things is before you sign on the dotted line. At this point, your best bet might be to just take what the seller has offered and consider it a learning experience. Unfortunately, it sounds like the "professionals" you hired to protect you in such a scenario did not perform their designated duties as well as they should have.