The input data is based on OpenStreetMap (www.osm.org).

The file should have been generated similar to the following script:

//direct to osmosis
cd C:\Program Files (x86)\osmosis
//derive Munich region; Munich.poly is generated by QGis plugin
osmosis --read-pbf file=bayern.osm.pbf --bounding-polygon file=Munich.poly  --write-xml Munich.osm
//only keep the building objects
osmfilter Munich.osm --keep="building=" -o=MunichBuilding.osm
//only keep the tag of levels in other tags
osmfilter MunichBuilding.osm --keep-tags="all building= building:levels=" -o=MunichBuildingClean.osm

... then: 

use a specific QGIS plugin to "dissolve" building polygons.

... then:

export as geojson file