Nationwide didn't raise mine at all. They did have an inspector look at the finished product though.
I contacted them prior to having my chimney built and got there specs. When I gave them to my brick layer, he said his specs were more rigid than Nationwides. I think they do charge more for prefab though.
You have to really shop around and have your insurance broker reveiw newly enacted company services. When looking for there chimney regs, my broker found out that if I joined the Pa. Farm Bureau, I would get a 15% credit towards my Auto/ Homeowner rates. It cost me $75 to get $215 dollars worth of discounts.
Also have your agent look for discount waivers that you can sign on to. Several that I have in affect are, no trampolines, a list of certain viscious dogs and a limit on the companies liabilities if some one sues you do to mold exposure in your house. That was the funniest one, but I guess in certain parts of the country, mold exposure is the new wiplash.
I got good coverage and several real good discounts. Still seems like everytime theres a hurricane in Florida though, my rates inch up.