im just building mine as we speak 24ft x 20ft all timber if you ever need any cheap sheets or timber let me know i always come across it.
ill get some pics of mine soon.
because i live in a listed building i was so limited what i can do but mine has cost very little to build just the roof was the most expensive i got a mate to sarnafil it as felting would be annoying doing every 3 years and i can tile it as i have a shallow pitch.

what is the width and length of the double garage section