Since the beginning of the 21st century, the future of religious heritage has provoked consideration within the fields of law, history, architecture and heritage. The origin of this problem is explained both by the decline in traditional worship practices, essentially Catholic in France, from the 1960s onwards, and by the Church’s lack of human and financial resources, which has resulted in the appearance of veritable "religious desert". This process, related to society’s secularisation, is expected to increase because of the likelihood of these two factors intensifying. If convents, monasteries, seminaries and other Catholic religious buildings have already been affected by this phenomenon, parish church buildings are now in turn losing their original function. Quebec has also experienced this phenomenon, but its historical and legal contexts have accelerated the transformation of churches which in turn resulted in the establishment of a "churches Plan" aiming at preserving church buildings by converting them.In this thesis, that pioneering programme is used in relation to the Lyons Saint-Etienne urban area, as a lens through which to read the French situation and as a tool for generating methods adapted to its specific context.The first part summarises the specific heritage and legal knowledge bases from Quebec and France concerning their parish churches, which is necessary for understanding the two contexts. The second part is an observational study, which defines the territory and creates an inventory of the corpus of research. We first provide an inventory of 429 parish churches within the territory studied here. From an analysis of their transformations, we propose three major typologies ("historical", "19th-century" and "20 h-century " churches), Using the cases of church conversions in ou corpus, we analyse the degree of compatibility between their previous worship use and their new uses, and then formulate hypotheses relating architectural interventions for adaptative reuse to restoration theories. The third part is an action-research interventional study. Three representative case studies from each church typology have been selected, in the town of Montarcher, Givors and Villeurbanne. For each case, a participatory approach has been set up with the municipality, inhabitants and associations, in order to propose reconversion scenarios adapted to local needs.This work shows that the demolition of parish churches, widely perceived as a common good, threatens the transmission of local identities. It therefore seems essential to start a global reflection on the evolution of this heritage, one which takes into account territorial issues, citizens' demands and the architectural diversity of these buildings. We show that the architect, through both his sensitivity to the place and his technical, can play a central role in implementing of these reflection. / Since the beginning of the 21st century, the future of religious heritage has provoked consideration within the fields of law, history, architecture and heritage. The origin of this problem is explained both by the decline in traditional worship practices, essentially Catholic in France, from the 1960s onwards, and by the Church’s lack of human and financial resources, which has resulted in the appearance of veritable "religious desert". This process, related to society’s secularisation, is expected to increase because of the likelihood of these two factors intensifying. If convents, monasteries, seminaries and other Catholic religious buildings have already been affected by this phenomenon, parish church buildings are now in turn losing their original function. Quebec has also experienced this phenomenon, but its historical and legal contexts have accelerated the transformation of churches which in turn resulted in the establishment of a "churches Plan" aiming at preserving church buildings by converting them.In this thesis, that pioneering programme is used in relation to the Lyons Saint-Etienne urban area, as a lens through which to read the French situation and as a tool for generating methods adapted to its specific context.The first part summarises the specific heritage and legal knowledge bases from Quebec and France concerning their parish churches, which is necessary for understanding the two contexts. The second part is an observational study, which defines the territory and creates an inventory of the corpus of research. We first provide an inventory of 429 parish churches within the territory studied here. From an analysis of their transformations, we propose three major typologies ("historical", "19th-century" and "20 h-century " churches), Using the cases of church conversions in ou corpus, we analyse the degree of compatibility between their previous worship use and their new uses, and then formulate hypotheses relating architectural interventions for adaptative reuse to restoration theories. The third part is an action-research interventional study. Three representative case studies from each church typology have been selected, in the town of Montarcher, Givors and Villeurbanne. For each case, a participatory approach has been set up with the municipality, inhabitants and associations, in order to propose reconversion scenarios adapted to local needs.This work shows that the demolition of parish churches, widely perceived as a common good, threatens the transmission of local identities. It therefore seems essential to start a global reflection on the evolution of this heritage, one which takes into account territorial issues, citizens' demands and the architectural diversity of these buildings. We show that the architect, through both his sensitivity to the place and his technical, can play a central role in implementing of these reflection.