In a previous life, I was employed by a large Architectural firm with a primarily residential focus that performed a lot of home design for home builders. One of the characteristics of creating home plans for builders was that they typically desired houses that had mirrored versions of the same plan. While this is done in other construction types, it is typical for most home plans.
Revit has provided a function to mirror the entire project without selecting all entities and using the mirror command as was often done in AutoCAD. The standard mirror command does not work for this and you should really only use the mirror command for 2D entities (annotation, line work, and other annotation).
To mirror the entire project, go to the Manage tab, then the Project Location panel. Pick the dropdown arrow next to Position and select the Mirror Project option.
Select the direction in which you desire to mirror the project. The options are:
- North to South
- Mirror line is East – West axis.
- East to West
- Mirror line is North – South axis.
- Northeast to Southwest
- Mirror line is Northwest – Southeast axis.
- Northwest to Southeast
- Mirror line is Northeast – Southwest axis.
The entire model will be mirrored in all views, including 3D entities and 2D entities.
Notes:
- Review location of text and dimensions as they will likely need to be repositioned.
- Text that is left justified will become right justified, and vice versa.
Applying Technology to Architecture 
I tried this in a project that had several families with connectors and noticed that performing this operation caused reversed the side of the family on which the connectors were located the other side of the mirrored equipment – as if the physical object itself had been mirrored. I then looked at the families and discovered that many of hte entities, including connectors, had not been locked to reference planes. All the more reason to be very careful when selecting families to use in a project that might need to be mirrored.
This was really Helpful! i used the mirror tool while having the whole project selected which resulted in multiple errors. you made my day!