Also check out this [link](http://www.globalquakemodel.org/media/publication/DATA-CAPTURE-GEM-Userguide-Footprint-Homogenous-Zones-201401-V01.pdf). It provides an in-depth step by step look at how to extract building footprints using both Arc Gis and QGIS with plugins Built-­up RECognition tool (BREC) and GRASS.