Merging NavMeshSurfaces

Hi,
I have a Terrain with it`s NavMeshSurface a instantiate some buildings on it and they have their own NavMeshSurfaces.

But the agents (who are instantiated later than the buildings) keep ignoring the latter, which results in agents being submerged into foundations(which do have colliders btw) just to walk on the level of the Terrain. I have tried some methods to solve this, but the only one that I got right was completely Building the NavMesh once more, which takes waaaay too much time to be useful.

So my question is:
What is the easiest way of adding the small NavMeshSurface of a building to the bigger NavMeshSurface of a Terrain?