I have a building box 3d model create from ketchup

How to I export feature include geometry is bottom face of model and propety is height of model from a 3d model?

The feature template

"type": "Feature",

   "properties": {},

   "geometry": {

    "type": "Polygon",

    "coordinates": c

     u

      Â

       y,

       x

      ],

      >

       y,

       x

      ]

     ]

    ]

   }