<p><strong>Abstract.</strong> In urban planning, a common unit of measurement for population density is the number of households per hectare. However, the actual size of the households is seldom considered, neither in 2D nor in 3D. This paper proposes a method to calculate the average size of the household in existing urban areas from available open data and to use it as a design parameter for new urban development. The proposed unit of measurement includes outdoor and indoor spaces, the latter comprising both residential and non-residential spaces. As a test case, a to-be-planned neighbourhood in Amsterdam, called Sloterdijk One, was chosen. First, the sizes of “typical” households, as well as a series of KPIs, were computed in existing neighbourhoods of Amsterdam, based on their similarities with the envisioned Sloterdijk One plan. Successively, the resulting size of the household was used as a design parameter in a custom-made tool to generate semi-automatically several design proposals for Sloterdijk One. Additionally, each proposal can be exported as a CityGML model and visualised using web-based virtual globes, too. Significant differences among the resulting proposals based on this new unit of measurement were encountered, meaning that the average size of a household plays indeed a major role.</p>