I have validated the GML data in the CityDoctor. It found the error "GE_P_NON_PLANAR_POLYGON_DISTANCE_PLANE" in nearly all of the buildings. Is there a way to fix such an error using FME?

@gulce​ Hi there, I believe this error occurs when polygons (specifically their verticies) do not fall within some specified tolerance distance of a reference plane. Without knowing the specifics of your dataset, you might be able to catch these non-planar polygons with the GeometryValidator transformer set to detect Non-Planar surfaces, or set to detect those features which are non-OGC Compliant.

Here's a Community Question that has some relevant information on workflow options that might prove helpful here. Take a look through the various responses therein for some tips on correcting non-planar surfaces with the GeometryValidator.