I am having a 30x30 built right now. Look at the specs, and get many proposals. I was suprised at the price differences between the buildings with the same specs. I got 6 proposals, and am going with these guys,
http://www.fettervillesales.com/ and am getting this style, only with the lean to on the right side.
I'll pour the floor after the building is up. I am probably going to extend it back another 10 or 15' eventually, here in Harford county MD, you can only go half the sq/ft of the living space in your house.
The house I am building is an 1800 sq/ft rancher. I could claim the 1800 sq/ft in the basement, but my tax rate would go up.
The only other option would be to build a breezeway attaching it to my house, then it's classified as an addition, and you can go bigger.
Total installed cost for my 30'x30' with an 8'x30' leanto, sliding doors front and back, 2 windows and a ridgevent is $15,700. Hopefully I can get the cocnrete poured for 3 grand or so.