A large share of a nation’s capital stock is in its buildings. Kenya’s largest city, Nairobi, is rapidly reconfiguring its built capital and doing so in astonishing quantities – a process that comes overwhelmingly with issues of land market informality. Interesting questions arise in such a context. How does the built environment of a city evolve? Is there a role for slum-style building technology in effective urban development? What are the costs of informality and poor institutions in land markets? In this paper we provide both novel empirics and develop a theoretical framework to answer these types of questions.