Also check out this GEM User guide: Geospatial tools for building footprint and homogeneous zone extraction from imageryGEM User guide: Geospatial tools for building footprint and homogeneous zone extraction from imagery.
It provides an in-depth step by step look at how to extract building footprints using both ArcGIS DEsktop and QGIS with plugins Built-up RECognition tool (BREC) and GRASS.