GROTON — In the face of declining home sales and consequent real estate values, the Board of Assessors has begun crunching numbers with the aim of making the town’s tax-collection process as fair as it can possibly be.

Taking into account property on high traffic roads, separate land and house values, different parts of town and even differences in homes between neighborhoods, and the difference in square footage from home to home, assessors have created a database that will enable them to move away from a current system that groups property in town according to sections such as West Groton, East Groton, downtown, or Lost Lake to one that groups similar properties together within particular neighborhoods.

In the wake of taxpayer unrest that erupted some months ago when property values were adjusted upward for lots deemed to be “shoreline” properties and unhappiness that is sure to come when taxes fail to come down even amid the worst real-estate environment in generations, assessors have spent countless hours in research and reconciling their findings with requirements mandated by the state’s Department of Revenue.

The immediate problem assessors seek to correct with their work is to reduce, if not eliminate, imbalances that arise in the current system that groups all property into single sections of town. For instance, taken as a single unit, West Groton might have a number of modest single family homes built in the 1960s and 70s when the average size of a house came to only 1,200 square feet, and modern “McMansions” whose usual square footage often comes to 3,000 square feet or more. Such large homes have the effect of skewing the average value of a home in a particular area, such as West Groton, to the higher end and away from the actual lower valuations due to smaller, more numerous homes.

To solve that problem, a more detailed database is needed so that sections such as West Groton could be further divided into neighborhoods of similarly sized homes and an average valuation created that is closer to reality. This would allow property owners to pay a tax more in keeping with the actual size of their homes.

Adding timeliness to the project is the continued sluggishness in real-estate sales across the state and the nation with a stunning 33.4 percent drop in the sales of single family homes in the Greater Lowell area last June compared with the same time the previous year.

Factors contributing to the fall in sales is the continued slowdown in the economy and the end of stimulus-based tax credits for first-time homebuyers. In addition, many homeowners find themselves with property worth less than their mortgages preventing them from selling at the lower prices.

Groton, however, seems somewhat immune from those circumstances with a high number of building permits for new homes having been issued since the beginning of the year and home sales and prices remaining steady.

That said, home values have declined somewhat and property owners are justified in wondering why their valuations have not been reduced as a result.

The answer, said Board of Assessors member Rena Swezey, is that the sales information the Assessors Office uses to set valuations and thus the real-estate tax paid by property owners is generally 18 months old and based on data supplied by the DOR for the years 2009-10.

It could be worse: According to DOR regulations, assessors need only revaluate local property values every three years. In that respect, Groton is actually doing pretty good by its residents.

Furthermore, property owners who wonder why their taxes keep going up while the value of their homes go down, are reminded that the tax rate is set not by the assessor’s office or the BOA, but the Board of Selectmen and ultimately, themselves.

The tax rate is based not on valuations but on the size of the town’s budget, which is formulated by the town manager and reviewed and approved by the Finance Committee and Board of Selectmen. But that is not the end of it. Once accepted by the selectmen, the budget next goes to Town Meeting for review and approval by residents.

When residents vote to approve a budget, the money to pay for it must be raised through taxes and if spending increases from one year to the next, more money must be raised from real-estate taxes. To that end, even if the value of real estate in town decreases, enough money must still be raised to cover budgetary expenses approved by residents.

Thus, a tax rate based on each $1,000 that a property is worth must be increased enough to cover the approved spending. The BOA does the numbers to figure out what that increase ought to be and makes the recommendation to selectmen who then vote to set the new rate.

Currently, the tax rate for real estate in Groton for 2010 is set at $15.78 per $1,000 of valuation.