 According to the seismic hazard, the municipality is in zone 4 (irrelevant seismicity), as established by PCM ordinance 3274 of 20/03/2003.The urban fabric of San Giorgio su Legnano has developed around the old town.Then, gradually, between 1914 and 1958, San Giorgio was the subject of a marked urbanization that led to the occupation of most of the area of the municipality.