"an extensive list" is correct and usable in written English. It is typically used to refer to a list of items that is long and comprehensive. For example: "We compiled an extensive list of potential solutions to the issue.".
It's already an extensive list.
They gave an extensive list of recommendations.
"We've got an extensive list of targets," he replied.
Each chapter contains an extensive list of resources.
Ixora offers an extensive list of teas, including two pricey white selections, peony and silver needle.
They plan to have an extensive list of Cognacs and to serve country French food.
But Mr. DeWind's interests extended far beyond tax codes to an extensive list of civic activities.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union