Cost to add Shed Dormer

[Jason Hatfield]

Hey BP!

I'm considering a deal on a small 2/1 Cape Cod in Chester County Pennsylvania.  Property is currently "legally" listed as 640 square feet but the owner is renting it as a 3/1 using the finished attic as the master bedroom.

If I put in a shed dormer on one side or even both then I can make it a master suite with a full bath and walk in closet. This could potentially double the legal square footage.  

My primary question is what is the ball park cost per LF to do this and what does the cost include?

Thanks!