The Federal Energy Regulatory Commission approved a permit for the pipeline in December 2014 on condition of getting a state water quality permit and completing work by December 2016. But New York's Department of Environmental Conservation denied the state permit in April, saying it failed to meet standards to protect streams and other water resources.
The company has appealed the permit denial to the U.S. Circuit Court of Appeals, arguing that the state's refusal is arbitrary and capricious.
In a letter to FERC on Friday, the developers asked to have the completion time extended to December 2018 while it pursues legal action.