In 2005, the Ontario government passed the Places to Grow Act and the Greenbelt Act, both major changes in land use policy designed to preserve greenspaces and combat urban sprawl in the Greater Golden Horseshoe, Canada's largest conurbation. This article examines the actors, actor beliefs, and inter‐actor alliances in the southern Ontario land use policy subsystem from the perspective of the Advocacy Coalition Framework (ACF). Specifically, this paper undertakes an empirical examination of the ACF's Belief Homophily Hypothesis, which holds that inter‐actor alliances form on the basis of shared policy‐relevant beliefs, creating advocacy coalitions. The analysis finds strong evidence of three advocacy coalitions in the policy subsystem—an agricultural coalition, an environmentalist coalition, and a developers' coalition—as predicted by the hypothesis. However, it also finds equally strong evidence of a cross‐coalition coordination network of peak organizations, something not predicted by the Belief Homophily Hypothesis, and in need of explanation within the ACF.