The historic buildings that now form our architectural heritage consist of structures that were built with the resources available at the time. Greisch office is often brought in to work on these buildings, whether for purely technical questions or alongside architects when the most complex decisions have to be made. Whether this involves old structures that need to be consolidated with new methods and techniques (a church belfry), or buildings that have to be restored and respected in their entirety, including their technical and expressive qualities (the personal house of Victor Horta, now the Horta Museum), or buildings that are to be given a new purpose by introducing new structures while retaining the existing ones (the former mills at Beez, which now house the archives of the Walloon Region, la Maison de la Bellone), a series of different attitudes can be adopted. All of these require a rigorous evaluation of the existing condition of the building and a philosophy for its conservation. This is drawn up jointly with the architects, and identifies the value of existing features in order to determine to what extent they can be preserved and remain significant.

Down the years, Greisch office has carried out a large number of missions relating to buildings of a historic character, and has developed extensive experience in this type of work. The introduction of new technologies into ancient buildings shall be carried out with precautions that anticipate their evolution and interaction over time, and must find a form of expression in harmony with the existing structures and fit in with the history of the building. Modern adaptations and alterations inside ancient buildings to extend their life may lead to research being carried out to conserve the existing structures, while introducing new ones (as in the restoration of the theatre in Namur). Collaboration with the architects and people responsible for the conservation enables the basic decisions to be defined, and to take greater liberty or adhere strictly to the original character of the building.