In this paper we study how the urban structure affects two environmental outcomes : greenhouse gas emissions from commuting and the conversion of agricultural/natural land ressources. The urban structure is characterized by its degree of polycentrism and by the share of urban and peri-urban dwellers. Polycentrism may lead to opposite effects on these indicators as lower commuting costs in secondary centers lead to lower land rents that may allow households to consume more residential land; furthermore commuting lenght may decrease, if workers commute to the closest business district, inducing less GHG emissions. The environmental impact of the urban structure is very dependant on residential lot sizes, fixed within zones but different between them. Then we address the implementation of two types of instruments to comply with an exogenous GHG target, a commuting tax/subsidy and a communication tax/subsidy for firms established in the secondary center.