The first stage of the huge project to modernise the inter-municipal hospital split between the Poissy and Saint-Germain-en-Laye sites, a new 19,000 m², seven-floor building will include emergency services, adult resuscitation, cardiology and neurovascular intensive care units, a level III maternity unit, neonatal, neonatal resuscitation, fertility services, genetics and hospitalisation rooms. The development, being carried out while the site is occupied, has proved complex as the new building is being grafted onto the heart of the old site, within immediate proximity of critical services such as MRI and recovery rooms. By using BIM (Building Information Modelling) and the FUZOR tool, users have been able to immerse themselves in their future premises by means of virtual reality, interact with execution models and check the technical facilities, as well as familiarise themselves with the new spaces. All of this has saved valuable time. The micro-installation of terminals was quickly validated and the changes requested by users were made in time prior to execution.