Adding buffers layers before adding layers.
snappyHexMeshDict has a top-level entry describing the type of meshing:
// Type of mesh generation:
// - castellated (default)
// - castellatedBufferLayer
type castellatedBufferLayer;See e.g. tutorial mesh/snappyHexMesh/rotated_block
With default setting there should be no change in the mesh generation. Only if the face-splitting during snapping (nFaceSplitInterval) is used are there slight differences due to a fix in the logic.