If I am understanding you correctly, you purchased the home without having the certificates of occupancy in place for the playroom and pool? Were you working with an agent who represented you in the transaction? Did the attorney representing you advise that the taxes would increase once you applied to the town for the Certificates of Occupancy? Did you know that the certificates of occupancy were not in place? If not, You may wish to revisit with them both to discuss.
However, the most important thing to do now is to try and correct the situation.
At this point, it is correct that you can attempt to grieve your taxes. Contact the Town of Islip in order to get the correct information regarding the specific dates that this must be done. You have three options:
Complete the paperwork and file the greivance yourself.
Hire a Professional company that deals in tax greivances.
Hire an attorney, who may cost more, but may provide more personalized service.
Also correct, is to apply for the star exemption which will help lower your taxes due, although has nothing to do with the decreased value of your home,which is dictated by the supply and demand of the current Real Estate Market.
Best of luck to you,
Dorothea "Dot" Salvo
Keller Williams Realty
Good Luck & Have A Very Happy Holidays!!