If the DTM model is on a non-flat terrain model, proceed as follows:

1. Stitch together all the building shapes to get a SmartSurface.
2. Select the triangles that make up the bottom of the building and stitch together using the Stitch Surfaces tool.
3. Use the Trim Surfaces tool with options as below.
   
   
   
4. Stitch together the two SmartSurfaces.