The £7 million development is expected to be completed by the end of the year, adding an extra 30 jobs to its current 90-strong workforce. Payne and Mills left BMW to buy Benham in 2005.
A new 21-car showroom and 21-bay service workshop for BMW is being built alongside Benham's existing site in Raby Street. That in turn will be refurbished in early 2009 as a standalone Mini showroom with space for 12 cars and an eight bay workshop.
Once completed, the site will have 200 metres of frontage on Wolverhampton's ring road.
Mills said: "Our present site is one third of the size it needs to be to cope with our increased sales and services. We sell over two thousand cars a year and deal with 60 service customers every day."
Talbot Construction and MDG Architects are involved in the project.