The rules are a combination of the state's shoreland protection act and your town's zoning ordinances. I've tried many times to apply common sense to what I read in the law but it never seems to work. What I think the law says is never actually what it says.
My suggestion is that you talk to a permitting expert. Its worth the consultation fee (if any) because then you get an answer you can be sure of.
I typically use:
http://nhenvironmentalconsultants.com/about_us
I know watermark is also the 800lb gorilla for winnipesaukee permitting so I'm sure they could help too!