If that information is WRONG by at least 10%, you can get a refund for the past 3 years, (If you Ask)
Then get a local realtor or an Appraiser to pull comparable sales. Not Sales, COMPARABLE SALES. (Homes similar to yours that have sold in the past 2 years). Then do the calculations for a new value estimate.
Follow the rules your County or local tax authority has for your appeal. You can do this yourself, until they want to play hard ball. Then get a qualified Real Estate Appraiser to help. After that you may need to get your Attorney involved. see http://www.FairTaxAppraiser.com for more information.
The next step is to present a grievance to the local assessment board of review. The taxpayer who wants to take matters beyond that point may go to court or take advantage of NEW YORK STATE'S simple small claims procedure intended for review of grievances on residential property. When other actions have failed, a property owner can bring a CERTIORARI PROCEEDING asking for judicial review by a higher court, board, or tribunal. A CERTIORARI PROCEEDING falls under the category of a ARTICLE 78 PROCEEDING; this is the appeal process available to a private landowner when a governmental body renders a negative determination on that landowner.
On the protest form there are just a few lines to explain the reason for your protest. You may be able to attach something additional but there is a game to be played here. Put down good reasons but don't reveal your entire hand. The reason is that seldom is your protest going to be granted and the next step is a hearing. Most people don't prepare well for the hearing and the Assessor won't either if you put down just a few factors on your protest.
The Assessor is bound to value your property by the "market approach". What would the average buyer be willing to pay? Right now property values are in the tank. There is a snapshot of time during which the Assessor has to make their appraisal. Usually that was some time ago. Like the summer of 2008.
Assuming that the Assessor doesn't act on your protest (remember they have never seen your house) you need to prepare for a hearing. You will not be the only person protesting. You will have about 30 minutes to present your case to a hearing officer in a small conference room. There will be a representative of the Assessors office present but they probably just got out of a previous hearing, are swamped with cases and don't know much about your case except what they can absorb in about 10 minutes. It is very structured at the Assessor's office and they expect nothing from you except a verbal whining to the hearing officer.
The hearing officer is usually an attorney hired by the county to mediate protests. So think like an attorney.and prepare, prepare, prepare.
If you can hire a certified independant appraiser to walk through your house and the grounds. They usually type up a report so make two copies (clearly identified as such) and take your original (never give that up) and the copies to the hearing. Be sure to point out every little item in your house to your appraiser during the walk through. Make sure they measure your lot (it will take at least a 100 ft tape), measure your house (the exterior is OK even though it doesn't accurately measure the interior space), point out any painting that needs to be done, siding cracked, decks that need restaining and sealing, roofs that are older, grass that is worn or weedy, whether you have a sprinkler system or not, cracked sidewalks, etc.
On the interior point out worn carpet, the kitchen sink you have been meaning to replace because the enamel finish is about gone, garbage disposal, condition of sink fixtures, the kitchen cabinets, the hole in the vinyl flooring, the hole the doorknob made in the wall, paint condition, wall tile conditon, how may bedrooms you have and their size (you should have a floorplan with dimensions), make sure the electrical fixtures are noted, the carpet that needs replacement or cleaning, any fence that needs work.
Then you should take photographs of everything. Even if you have to buy a disposable camera. Get close ups of everything that isn't brand new. Don't depend on just the flash to light things up. Take pictures in day time, use existing lighting (turn it on) and a flood light held in any fixture with a reflector (held by someone else to light up a room). You need 3 copies of these pictures. One for you, one for the hearing officer, and one for the assessor's representative. Write up a list of things that support your side of the arguement (like all the deferred maintenance, concrete that needs replacing, painting, carpets, etc. Make three copies of your list--distributed as the pictures are but only during the hearing (Usually called Board of Equalization Hearings)
Practice a quick presentation so it doesn't appear that you just discoverd everything 10 minutes ago. The Assessors office rep will be totally surprised when you start talking and shoving written reports, pictures etc. across the table.
Make sure that whatever is listed as part of your house is accurate (they charged me for a non existent bathroom for 15 years) They also put a rating of the quality of finish on the rooms in your house. Example: Granite or Marble countertops are a high grade finish than Formica. Ceramic floor tile is higher than linoleum or vinyl. Hot water heat (using a boiler) is higher than hot forced air. Air Conditioning is higher than none or a swamp cooler. Concrete driveways are more than asphalt driveways. Etc, etc.
The assessor will compare your house to "comps" to determine value they haven't seen.