The increasing impacts of built-up land expansion on sustainable development have heightened the use of spatial planning as a policy tool to contain built-up land expansion. However, causal evidence for the effect of spatial planning on built-up land expansion has largely remained unexplored. In this study, we used a difference-in-difference model with propensity score matching to estimate the average and annual effect of built-up land zoning (subsequently called zoning) on built-up land expansion in Zhangzhou City, China between 2010 and 2020. Results on the average effect show that zoning was effective in containing built-up land expansion. Specifically, zoning prevented 27.02 km² of built-up land expansion outside the development-permitted zones between 2010 and 2020, which accounts for 32.46% of the observed built-up land expansion outside the development-permitted zones. We found a time-lag effect, with zoning starting to have an effect after 2013. Furthermore, zoning became ineffective in containing built-up land expansion at the end of plan implementation. Based on our findings, we recommend that future evaluations of the effect of spatial planning on land-use change use causal inference and that they explore the influence of time on the effect of plans in greater detail.