Past assessments of the carbon dioxide emissions attributable to buildings in Taiwan generally employed conversion formulas based on the floor area of legal buildings, and did not perform any quantitative assessments of illegal building space. As a consequence, this study builds on past research and places special emphasis on the effect of building violations. The study employed building violation data was derived from inspections performed by Tainan City Government from 2001 to 2007, a classification of illegal structure type, and environmental load analysis to derive the carbon dioxide emissions of four major types of building violations in Tainan City. It is found that reinforced concrete and light steel scaffolded concrete accounted for a combined share of 89% of all illegal structures in the city. In the floor area of illegal structures built each year in Tainan City accounts for roughly 5.6% of the floor area of authorized construction during the same year, but this quantity has been neglected in past assessments of building life cycle environmental load.