The study concentrates on residential areas which are mainly composed by detached houses to analyse the underlying economic and socio-political mechanisms of the densification dynamics of this areas. The research explores the hypothesis that the densification of residential neighbourhoods dominated by detached houses is primarily initiated by private actors (property developers, developer contractors, households/ private individuals, land owners), especially in areas with high value residential markets. The paper offers an in-depth analysis of the socio-political relations observed in the densification processes in suburban neighbourhoods of two municipalities of the Paris-City region. The first, Magny-les-Hameaux, introduced a discretionary densification policy in its suburban neighbourhoods; while the second, Noisy-le-Grand, is implementing an interventionist policy of radical densification on several of its detached housing districts. In studying these two differing processes and the policies linked to them, and exploring the extent to which land and property market cycles affect the fabric of the city, we discuss the possible orientations of a public policy in relation to a process which first results from a market mechanism. In doing so, we explore the extent to which the local implementation of compact city policies and their associated processes represent the interests and objectives of political and economic elites in controlling, managing or seizing local land value and planning systems for their own benefit. In particular, we study the shifting influence of the policy instruments used to regulate the densification dynamics on the changing role of public authorities and their relationship with market actors.