This paper articulates an urban design logic of informal settlement. While there are many different morphological outcomes of informal settlement processes, there are common patterns of expansion, intensification, and design of public space networks. Unlike formal urban design where the street network comes first, informal settlement involves a complex co-emergence of buildings and streets. Based on a global range of morphogenic studies, this paper articulates the spatial logic through which street/lane networks emerge incrementally in conjunction with building and plotting. Morphogenic mapping shows how settlements expand and intensify, how they stabilize or ‘overdevelop’. Understanding this logic requires new concepts such as ‘incremental gridding’, ‘back-to-back plotting’, and ‘interface creep’. This work expands our knowledge of how informal urban design works, and how the emergence of ‘slum’ conditions might be avoided. A better understanding of this morphogenic logic is the basis for any effective approach to in situ upgrading.