If you are planing to use online estimating tools like the one available at Zillow, you should take these estimates with the grain of salt. Here what one correspondent at Boston Globe writes:

For all their sophisticated presentation, the new websites are still struggling with a common problem: how to get accurate property information, beyond what’s publicly available from common sources. For example, Zillow said Russell’s condo had two bedrooms and one bathroom, when in fact it has three bedrooms and two baths, and also omitted valuable off-street deeded parking. Those discrepancies accounted for much of the difference between Zillow’s most recent $504,398 estimated value of Russell’s Cambridge condominium and his Make Me Move price of $800,000, Russell said.