Carport - cost and benefit

I finally made the connection of my idea for the design of the carport/workshop we are thinking about for the back. The inside would resemble the front of our house. My current thought is to place it in the southwest corner. in the photo of the back yard that would be in the right corner. The alley side would resemble a garage with a garage door. The structure inside the fence would be an open carport looking something like our front porch. The work shop would be on the right side and enclosed. Our "Gold Plan" would include replacing the windows on the house. The existing windows from the house would be used for the work shop. We were sitting on the porch last night and I realized that I want the carport to resemble the front of the house with the low retaining wall with concrete caps and a wide opening, and columns, leaving the carport side open. The workshop would be under the roof and consist of walls with the large windows. I was amazed that I had not made that connection and I was amazed at the connection, for now that makes my ideas clear, and the plan workable. Now the thing to do is to work up some detailed drawings. With the drawings I must determine to cost of constructing the carport and get a realistic, professional appraisal as to the amount of value this improvement would add to the property. We have no money available at the present for this addition, but that should not hinder me from going forward with the plans. Something will arise to make this project a reality.