Arun could have missed out on millions of pounds of developer contributions to fund vital infrastructure improvements.
West Sussex County Council adopted the Total Access Demand (TAD) system in 2003, signing up all planning authorities across the county apart from Arun District Council.
Figures show the system had secured £38.5 million from developers since.
Replying to a written question from Liberal Democrat councillor Dr James Walsh at full council last Wednesday, county cabinet member for highways and transport Bob Lanzer said: “It was not adopted by Arun.
“Unfortunately, therefore, there has been no way for the County Council to seek contributions that could be used to fund improvements in the wider Arun area, for example, those currently proposed for the A259.”
Arun had not returned a comment at time of press.