This is extremely rare. The key, as most people point out, is the need for intention, and that is hard to come by in any random circumstantial community of people living in proximity by chance. That is why most intentional communities must advertise for members. They market their intent to attract members. This includes cohousing.
Natural law is a very versatile theory.
The general idea is to think of a particluar city as the environment for the community, some number of the residents of which will be interested in community, while the vast majority will not. The goal is to find ways to network those who do want to support and participate in an urban network community. This is my best thinking on the subject, and now constitutes my community building strategy. I have recently resigned from my cohousing work to focus on this NLC concept. I hope to keep this listserve apprised of our development.