Hello Dsbmomof3 and thanks for your post.
Architectural drawings and plans are the intellectual property of the architect that made the design. So while the homeowner may commission the plans and own a set for their home, unless the Roseanne homeowner AND the architect agree to give you the plans, the only other way to obtain a set is by purchasing a set from the architectural company or architect that designed the Roseanne home. You may be able to see a small version of the plans at the building department of your local government, but should you ask for a copy, you'll be told that the plans cannot be released without the approval of the architect. Typically, architects will only release copies to the homeowners who originally purchased the plans.
Talk with the Roseanne homeowner to see if you can obtain the name and contact information of the architect who made the designs for the home. Remember, building a home or addition in many municipalities requires both the skills and talents of an architect as well as that of a structural engineer, so just obtaining plans is unlikely to be enough to obtain building permits from the local government.
Grace Morioka, SRES, e-Pro
Area Pro Realty
San Jose, CA