According to a new study by CE Delft, construction of the new airport planned near Nantes, l’Aéroport du Grand Ouest, will lead to a decrease in economic welfare, while improving the current airport, Nantes Atlantique, would yield net economic benefits. These findings are contrary to the analysis presented in the public consultation held in 2006. CE Delft was commissioned by CéDpa, the organisation of elected officials opposed to l’Aéroport du Grand Ouest, to analyse the social costs and benefits of the new airport and of improvements to Nantes Atlantique. It has found that the existing analysis contains a number of errors which, when corrected, result in a significant decrease in the benefits and an increase in the costs of the new airport. The main benefit cited, travel time savings of those using it, had been erroneously calculated. Using standardised values for the cost of time, CE Delft finds that the actual benefits are two-thirds lower. Moreover, the public consultation documents failed to include the costs of the biodiversity loss incurred in transforming a rich natural area into an airport. The current airport, Nantes Atlantique, can be improved to increase its capacity. Compared with construction of a new airport, the economic benefit would be at least EUR 200 million. CE Delft has a long track record in social cost-benefit analyses of infrastructure projects. It has extensive experience in the aviation sector and has reviewed plans for the third runway at Heathrow (London), expansion of Schiphol Airport (Amsterdam) and other such projects.