The new million dollar question: When it comes to revitalizing cities, or finding places to put people, is it better to build or rebuild? The U.K. has opted in this case for the former, and has given the go-ahead to build four "eco-towns" with as many as 100,000 new houses. These houses will be built from scratch and will in theory be set up to run as sustainably as modern technology allows. Which is to say, quite. Though it's unclear so far if it'll work out that way.It sounds great on paper (it always does), but it's got lots of opposition. One catch is that there are 800,000 empty houses in England that many locals would like to see put to use. "Why build 100,000 new ones?" goes the logic. Well, couple reasons: 1) It can cost a lot more to rehab an old house to make it energy efficient. 2) It doesn't make for as good a PR/tourism/politicky campaign.To be fair, if this is done right, it could epitomize smart development. We've written a bunch about fixing cities as well as building them from scratch, and the pitfalls associated with both. I'm still on the fence. I think it's all about execution-the proof is in the pudding. What do you think? Are there examples of smart development you can point to? Do you think rehab is always the way to go?