In recent years, the concept of compact cities has permeated the sustainability discourse under the premise that compact high density developments can effectively reduce car use levels and promote use of alternative modes such as transit and non-motorized means. However, these arguments hinge on the existence of a true causal mechanism between built environment and travel behavior. Using panel data from a survey on new movers to a high density mixed use development in the Kashiwanoha area, Chiba prefecture, several models are estimated to test the effect of changes in the built environment on activity frequency by mode. Findings suggest that even after controlling for residential self-selection, the built environment exerts a significant effect on activity frequency for some activity types such as shopping and eating-out conditional on travel modes. Mode substitution effects were observed between frequencies of nearby activities reached by non-motorized means and faraway activities reached by car given changes in accessibility levels around home location. Asymmetric effects of changes in car ownership on activity frequency were also identified. Findings provide a good insight on the potential effects of retrofitting low density suburban areas through densification and land use mixes, an issue of critical importance in the context of rapidly ageing and depopulating cities and regions in Japan.