Dear M & m,
As a Realtor and Civil Engineer, following will be your approximate costs:
Architectural and Structural Design: $3,500-$4,500
Permits and Township fees: $450
Cost of Construction: Minimum of $120- $150/SFT. If any contractor is doing the work cheaper than this, I will recommend to double check with other contractors. There are times when Contractor's take a job at a low price and cannot finish the work.
As far as return is concerned, I do not think you should consider any return on that with the exception of an additonal bedrm with full bath. I, Myself is not a big fan of additons.
Remember one thing, when you re-sale this house, the age of the house will still be the original age built and not the age when you did the addition.
For any further questions, Please contact me at my cell# 732-277-9321.
P.S. Search properties on your own at www.vishalmaher.com