I have a couple of questions: do you mean, that in your opinion your house is worth more than the appraisal and assessment values that came in? When was this done? When you say "it might be developers dream", does it mean that house needs work / updating? Or is it in overall very good condition, but has potential of expansion?
Also, how was the appraisal done? In some cases the appraiser only looks at the house from outside, without stepping in.
If you are planning to sell, market value is determined by what today's buyers are willing to pay for similar property. You'd need to talk to an agent about recent sold statistics, and a good agent would be able to go through them with you, explaining why one sold for more than the other, and where your property is in that equation.
Some properties are unique, and comparison to others on the market is not that easy, but it can be done. It might also take little more time to sell, because you'd be looking for a certain type of buyer who would appreciate all that this unique property has to offer. In that case, marketing should be adjusted to create an extensive internet exposure, to catch most wide pool of buyers. (say, relocating buyers who are currently still living in a different state and can only search MA homes online)... more