Key Messages Secondary suites play a vital, but poorly understood, role in the urban rental market. Many suites are illegal in one way or another and evade oversight. Recently, Canadian municipalities have begun to treat secondary suites much more favourably.

This study developed and tested an initial tool for the systematic observation of secondary units at the neighbourhood scale by utilizing an inventory checklist method inspired by studies in the fields of health geography and criminology. Single‐family dwellings with secondary units were found to have one or more of 18 visual characteristics as outlined in the proposed tool. In the neighbourhoods where the tool was applied it was found that, within the Canadian context, urban neighbourhoods contain a higher proportion and density of secondary units than newer, suburban areas. Indicators that emerged as most prevalent included more than one mailbox per dwelling, unit numbers on single‐family dwellings, more than one electric meter, and lawns converted for parking of multiple vehicles. Local and regional variations in housing stock characteristics and local housing markets make the standardization of such a tool problematic. However, this method does present a place‐based approach that provides a subtle means of understanding the unique attributes, and distribution, of this type of housing at a scale relevant to municipal planning decisions.