Informality is a global phenomenon entrenched in the Global South and increasingly emerging in the Global North. In dense and compact informal settlements commonly seen in the Global South, housing transformation to meet resident needs typically occurs through small-scale physical alterations and adaptations. Within this setting, the aim of this paper is to provide a substantive assessment of the dynamics of change and spatial complexities of housing in informal settlements by deconstructing the micro-morphology occurring in built and unbuilt spaces. This includes the space where housing and alleyways meet; namely, the public-private interface. The paper achieves this by; (i) defining a set of interface typologies that constitute the form of the domestic housing space pushing into alleyways by using the key variables of distance and proximity, (ii) quantifying the housing typologies so as to examine types, patterns and processes of transformation, and (iii) understanding the basic planning and regulatory framework in which households undertake these adaptations. The method used in this research is a case study of kampung Lebak Siliwangi in northern Bandung, Indonesia. The paper makes a major contribution to the literature in terms of identifying principles by which housing transformation practices occur through micro-morphology which is locally self-organised and incremental. Residents undertake little or no consultation with neighbours or local governance leaders when making changes, though they observe and replicate the micro-scale changes made to housing form by other residents. The boundaries of the built (housing) and unbuilt spaces (alleyway) are fluid due to ‘interface creep’ which allows housing to create new building lines, interface types and alleyway alignments. In this setting, the notion of ‘interface creep’ and the reliance of residents on small-scale distance and proximity changes are a key narrative of the paper as they allow adaptability, having strong ties to the way housing and alleyways are locally regulated, controlled and governed. On the other hand, this dependence threatens housing conditions, sociality, access, multifunctionality and economic modes of production in public spaces (the alleyway). The paper concludes with comments of the implications of the research that could be applied to theoretical and methodological frameworks for a deeper understanding of informal housing in the Global North.