Validity term of structural survey/engineering report

[Andrey Pak]

Plan to order a structural engineering study to determine if the house has sufficient foundation to add a couple rooms above the back. 

I am assuming (happy to be corrected) that I would need to break it into several phases: structural study, possible strengthening of foundation, architectural drawings for additions on top of the back, and then permits and construction.

With that I have a couple questions:

1. What is the range of underpinnings installed for a 60 linear ft (for simplicity 20+20+20)?

2. How long will the structural report be valid for? Can I refer to it in 2 years, 3 years, once I am ready to proceed with the works?

Appreciate both direct answers and general pointers/advice on how to minimize the errors.