Interurban-level focus during the last decade has shifted from the compact city towards a polycentric urban framework. The ability to define consistent urban structures and also link them with sustainability goals has been hindered by inconsistent evaluation methods for density and mixed functionality in a polycentric framework. The aim of this research is to test and combine various methods from these perspectives in order to define more reliable and consistent descriptions of urban structures. The methods used are spatial-density modelling using kernel convolution, a polycentric density estimation, and methods depicting mixed functionality and the association between density and mixed functionality. The empirical findings relate to planning goals at both national and international level. The study region is the municipality of Strangnas, within the Stockholm City Region since 1997. Results from the analysis reveal urban development towards further segregated land use and sprawl, as well as a decreasing link with a polycentric urban scheme. The methods developed for depicting urban form could be useful tools in the planning process and may reinforce the possibility for analysing links between urban form and sustainability aspects. This improved knowledge in turn could contribute towards formulating future planning principles.