Abstract: This paper reflects my interest in how policy and law are used to develop models for more just and equitable city living. Making use of a law in action rather than a law in books approach, I draw on socio-legal scholarship and urban studies to consider knowledge formats for planning law decision makers (PLDM) in relation to social and cultural impacts for proposals for significant land use and development. I ask how PLDM ‘know’ about social and cultural impacts and what role their own social capital plays in regard to this knowledge. I also explore the role of ‘social impact assessment’ (SIA), as a legal actor and knowledge instrument in the decision making process. I draw on US socio-legal theory to suggest models for how ‘social facts’, which carry a troubled history in common law jurisdictions, could be presented as evidence by the planning expert witness in courts and tribunals. It is anticipated that this research can contribute to planning law decision makers possessing more informed knowledge capital with which to make decisions for ‘good’ city planning.