The largest project we had our friend build was this greenhouse:
http://www.ana-white.com/2012/05/plans/barn-greenhouse
We changed the orientation of the roof panels to facilitate rain runoff, added a door and benches inside, and I’m pleased to say it’s worked great for us as a season extender. Total costs under $300 since our carpenter is a good scrounger for unwanted building materials. 🙂