Insurance Company has revalued my property without explanation

[Mike Girard]

I already posted this but I fear I overexplained. 

I recently got a notice that because of "certain features" the insurance company's inspector found when giving my property the once-over, the valuation of my fairly standard, one hundred year old Philly row house has gone from the figure given by every agent -- 300,000 -- to 440,000. The company, Nationwide, is intent on raising my premium and charging me retroactively for the three months of insurance I've had so far. My house is 1989 square feet in a city where construction of row houses costs about 125 per square foot. 

I'm stunned that they can do this without any explanation of what "features" provoked this revaluation. The only thing separating me from the other houses on the same street is a little smoker's balcony.  The house has none of the original detail as far as I can tell, apart from some small marble bits here and there. Nationwide has, incredibly, tasked the receptionist in the local office with all direct communication about this and other really weighty matters. No one with any discretion has said a word, including my local agent, and I wouldn't even know this were happening yet had I not requested a copy of the letter informing me of this which is scheduled to be sent on 12/7.

What, if anything, can I do about this. Seems to me I should have the right at least to a detailed, written rationale for the dramatic revaluation.