This paper describes our conceptualization of complexity in large infrastructure projects. Since complexity itself is an emergent concept that is hard to pin down, we focus on the relationship between various project features and properties associated with complexity such as difficulty, outcome variability and non-linearity, and (non) governability. We propose a combined structural and process-based theoretical framework for understanding contributors to complexity-the 'House of Project

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... ty' (HoPC). The formulation of the HoPC draws from a rich projects literature and is developed iteratively by first applying it to two trial samples and then to the main data set of 20 detailed case studies of infrastructure projects prepared for the IMEC study. A main contribution of this work is the conceptual distinction in the HoPC between 'inherent project features', 'architectural features', and their relationship with project outcomes and emergent properties-the 'ilities.' A second contribution is the separation of 'inherent features' into the technical and institutional domains, which are then developed in parallel fashion. A third contribution is to link complexity with the concept of project architecting. The HoPC can be generally extended to projects in the extractive industries, large manufacturing projects or other industrial megaprojects.