Moving in with a partner is a step in the process of institutionalising a romantic relationship, that is, establishing the relationship in such a way that it is more embedded in the social environment and more strongly regulated by social norms and mutual expectations. But under what circumstances do couples decide to establish a joint household? We study the transition to a coresidential partnership among couples in a ‘living-apart-together’ (LAT) relationship. We use data from the seven waves of the German Family Panel (pairfam) dataset (N = 2428 LAT relationships of men and women born in 1971–1973, 1981–1983, and 1991–1993). In order to analyse especially the partnership processes of younger adults, we focus on respondents in the age range of 18 to 42 in the time period between 2008 to 2016. Using discrete-time event-history models, we test hypotheses about the partners’ resources, relationship quality, degree of relationship institutionalisation, and exposure to different kinds of costs. Our results indicate that especially equality and institutionalisation factors, as well as the costs of moving and of starting to coreside, are associated with the decision to move in together. Resources are important for those LAT partners who are living in the parental household, while for LAT partners who have already left the parental home no effect of resources was found on the transition to a coresidential union.