Ours will be DIY apart from the roof. Partner's Dad is an enthusiastic joiner who reckons he can largely prefab something and then erect it over a few weekends.
Wasn't originally very interested in sedum but quite keen now.
I thought of pre-fabbing at one point, but i didn't really have the space and it seemed much simpler just building in situ in the end. It's great fun - I'd never really done anything like that before and i learned bucketloads. Still standing 2 years later, which is a plus.
I really wanted a sedum roof on mine, but I was running out of time and budget at that point (my office/shed was a total DIY)