Learn how to create cut out in system families like walls or floors in Revit.
Select Architecture –> Model in-place. Remember to choose the same category you want to create a cut out in. For example if I want to create cut-out in walls I choos WALLS category in the first step.
Model a void geometry like extrustion or sweep. Please remember that you can snap to the existing geometry. Sometimes is good to create a section view that we later can go to in order to see how this cutout works with an existing elements.
When the void form is modelled use Cut command in the Modify tab to cut the floor or wall or any system family. Without this step revit will not allow you to finish the model.
Finish the In-place component. There you go! You now have a cutout in the system families.