How to spawn a navmeshagent on a certain nav mesh?

To spawn agents on navmesh randomly, firstly you can use NavMesh.CalculateTrangulation() and then you can pick randomly from these triangles.

 NavMeshTriangulation Triangles = NavMesh.CalculateTriangulation();
 int RandomNumber = Random.Range(0, Triangles.vertices.Length);

To make them only spawn in the houses, you can put box colliders inside the houses and use a Physics.OverlapSphere on that location to make sure that location is inside the house, so therefore, inside the house collider. If not, cancel spawning.

For better reference, here's a video over NavMesh Triangulation and spawning enemies randomly, but it doesn't talk about spawning at specific zones as far as I know.