Tesla started designing the S very early, when they were a very, very small company.
Check out some of the earliest blogs on Tesla's website. Elon discusses the "master plan," and not much later early Model S work was described. There was a press release about Tesla buying high-end CAD software, one of the best available, for the Model S design work. This was a sign they were serious, and were going to use the latest industry tools and methods in design. Later, Tesla announced the had to put Model S design on hold, due to severe financial pressure. They just did not have enough income.
I would guess the start of Model S design was somewhere around 2008.
GSP