Despite a rich research literature addressing the rise of a genuine multilevel administration (MLA) in the European Union (EU), impacts on municipal bureaucracies remain understudied. This article maps how the MLA affects local bureaucracies, based on a multi-sited qualitative study of public procurement and environmental departments in 20 Norwegian and Swedish municipalities. We find that the studied municipal bureaucrats interact routinely and closely outside their home institutions through inter-municipal coordination arenas, shared work processes and peer learning, revealing a decentralised, horizontal pattern of integration. The analysis further identifies indirect forms of horizontal co-optation operating through leading cities and networked arrangements, beyond direct European Commission influence and control. These findings refine existing MLA research by demonstrating how compatible territorial problem-solving operates as a horizontal mechanism of administrative integration within the EU MLA. © 2026 The Author(s). JCMS: Journal of Common Market Studies published by University Association for Contemporary European Studies and John Wiley & Sons Ltd.