regiongroup produces the zones for zonal statistics operations.
now you need to do the zonal statistics
I notice that you have an incredibly large number of zones, suggesting that your input raster wasn't cleaned up and you probably have a whole load of zones that consist of 1 or 2 cells. You could use nibble to fill those in with adjoining values, but I suggest that it might be worthwhile getting a clean vector file of the building footprints and start the zonal statistics from that