This is a common question we get when it comes to hidden dry rot damage as dry can be under your siding without you knowing. Since dry rot can travel, there is no way to see the damage’s extent without opening the material up and seeing what is under the siding. I would be wary of any contractor who appears the know the extent of the affected area just by looking at the outer material only. There are many factors to how dry rot spreads, amount of time, moisture levels, type of product affected, how the area was constructed, building defects, and animal damage. So, with all these factors to make a proper diagnosis, the affected areas need to be inspected and seen.