My build challenges are always DON'T build something, or do something a particular style. My first house was the hole in the ground, expanded as I needed more room. My first 1.5.2 house was... you guessed it, a hole in the ground. So my challenge was to build a house that was NOT a hole in the ground. And I ended up with a fairly awesome one that slowly expanded into a miniature village of my own design. Of course, I was also spending lots of time in a hole in the ground mining, because I really don't like to mine automatically, so I felt like the challenge didn't quite workout. My latest house was... yeah, another hole in the ground on the mountain. But this time the challenge was to build the base as large as possible. Now all the processing is taking place about two hundred blocks away, the mining is under the ocean, and I built myself an underwater mansion to live in. So that worked out much better.