Rapid expansion of built-up land is widespread and often does not follow established zoning regulations. This non-conforming expansion of built-up land may exacerbate social and environmental problems and has emerged as an important sustainability concern. In this study, we first evaluated the non-conforming expansion of built-up land to zoning in Zhangzhou City, China from 2010 to 2020. Non-conforming expansion of built-up land accounted for 67.61% of the newly developed built-up land area. These non-conformances mainly were edge expansion, via conversions from arable land and forest to industrial/mining/transportation land. We then used spatial autoregressive models to estimate five types of peer effects on non-conforming built-up land expansion among local governments. The geographical, political, economic, geographical-economic, and political-economic peer effects significantly increased the expansion area of non-conforming built-up land among 307 villages. Our findings improve our understanding, from the perspective of inter-governmental interactions, of why the extent of built-up land often was beyond the permitted limits of zoning regulations. Our research also has policy implications and suggests constraining the peer effects of local government land-use behaviors that violate zoning regulations. We recommend the establishment of closer interdisciplinary collaborations between spatial planning and land-system science to address non-conforming land-use changes.