I believe the present tax assessment must be more than 15% different from provable present value for any town to entertain a tax appeal. Go on line and see the process explained in Essex county, the appeal office is in East Orange. I can give you the name of an appraiser who can explain it to you. His work and then your follow through after Jan 1 and before April 1, 2011 might save you money. Although GR was re-assessed in 2008-9, yours may have an error somehow. Good luck. Carl (see profle)
Speak to your local tax assessor first to see if you can file an appeal. Many towns only let you file and appeal when reassessing. Ask first before hiring anyone and spending money. The assessor should be able to guide you on the best course of action and if it is possible to get a reassessment. Most township officials are very accomadating and helpful. Just ask.
If your doing it by yourself contact your local tax assessor. I recommend hiring an experienced real estate appraiser that specializes in tax appeals since it takes a substantial amount of work and they are more likley to acheive the goal of lowering your taxes if its at all possible. Even though it cost money to pay the appraiser, in the long run it will have paid for itself if the taxes get lowered.