The Abatement Process in Massachusetts isn't overly complicated.  Let's talk about what they are, and what to do if you're thinking the town is asking a bit too much from your wallet.  As always, this is not legal advice about what to do, as I am not an attorney, but it should give you a good overview of how the system works, and give you insights as to what your next steps are. 

What is an Abatement and What is an Assessment?

I often find that folks aren't clear on the terms that are used by the town offices, and getting the vocabulary out of the way is important to de-mystifying the process. We'll start with the assessment.  A town's assessment is their stated value of your home.  You can read more about a town's assessment at the link, but there are some key things to keep in mind.  The first, and most important, is that an assessment is NOT A MARKET VALUE of your home, although it is often related to that value in some way.  When we say market value, we generally mean "today's value" but an assessment is often an estimated value in the past, often 6 months or even 24 months in the past. Also, an assessment is NOT AN APPRAISAL.  An appraisal is usually the process to get the market value. 

The reasons towns have assessments is to collect taxes.  An assessment is used to place a value on the property RELATIVE to other properties at that same point in time (which is in the past!).  It's goal is to be "fair", more than it is to be accurate, although it tries to be both.  

An Abatement is a formal request, by the Tax Payer, to not pay their taxes because they feel their assessment is to high.  Note, you can't appeal your taxes  - you can only appeal the amount that the town has assessed your home, which is the basis for how they calculate your taxes. 

How are Assessments Calculated?

Assessments are calculated, generally, by simple computer programs that most towns have in their offices.  Using recent sale data and property data as inputs, they calculate assessments for homes that haven't sold in a while. A good assessor, who tweaks the program for local variables, can have a very accurate model for the properties in the town.  But the system is far from perfect, which is why homeowners are allowed to file abatements.

How do I Know If My Assessment is Too High?

If you haven't bought your home in a long time, you may not know.  It is not easy to determine what your home is "actually" worth (and Zillow is often of minimal help). Although most homes are assessed accurate, some are not, and there are some warning signs you can watch. In my experience, properties that have a permanent material issue - like a really steep driveway - tend to be properly assessed around a purchase, but over time, tend to gravitate away from the correct assessment as they get muddled in with homes that do not have the same material issue.

Bottom line, be friends with your local realtor, and stay on top of your homes market value. Then watch your tax bill and watch the assessment number.  If the assessment is clearly over market value, it may be time to file an abatement.

When Should I File an Abatement?

Generally, the first thing you should do - before filing an abatement - is discuss with the town assessor the assessed value of your property, and understand their reasoning behind the valuation.  Hopefully, you have already researched your home valuation, and can provide counter evidence to the assessor.  Having an open, unofficial dialog with the assessor should be possible - but not all towns will be friendly about it. Be prepared, have good data, and a sound rational, and most assessors will hear you out at least. 

If, after hearing you out, they rule against you, then you should file the official Abatement paperwork with the town.  This formal application serves the town notice that you are serious about the issue, and will force the town to come up with research to validate their model's assessment.  Be prepared, once you file an abatement, they will probably ask to see the inside of your home, so if you're telling them it's a dump, and it isn't, that will not go over well.  In fact, there is certainly a risk that he determines your home is worth more than the assessment, so be sure of your numbers before inviting him to tea. 

Does Filing an Abatement Guarantee I Won't Have to Pay My Real Estate Taxes?

Don't be silly.  Death and Taxes are the two certainties in this world, and you'll be paying taxes.  However, if your home was over assessed, you'll be paying less taxes.  Here's an example.  

The town does it's annual re-calculation, and determines that your home is worth 700,000.  Now, you happen to know that your neighbors house, which has the same floor plan and size, recently sold at 625,000.  There are probably differences between your home and your neighbor's, but not 75,000.  Armed with the public record sale of your neighbor's house, you head to the assessor and chat with him.  He comes over, and agrees - your home is about the same, and reduces your assessment to 625,000.  Yeah! But you still have to pay taxes on the $625,000 assessment.  You're bill is down 12% or so, but you're still paying 88% of that original tax bill. 

What if the Assessor overturns my Abatement and I still think they are Wrong?

You can appeal to the state, who will look at the assessors data and yours and make a final ruling.  The state board is the Appellate Tax Board, and their information can be found here: http://www.mass.gov/anf/hearings-and-appeals/oversight-agencies/atb/.