The 2-acre site is close to its current premises and it plans to develop a seven-bay workshop with an ATF lane.
Other services will include tachograph fitment and calibration as well as steam cleaning.
The dealer is also looking to acquire a retail van sales and service centre with showroom elsewhere in Southampton.
In May Marshall Motor Holdings (MMH) completed the £107m acquisition of Pentagon’s former parent, the multi-franchise Ridgeway Group.
Pentagon, which currently has branches in Andover, Fareham, Poole and Southampton (Nursling), has been led since 2009 by dealer principal Gary Brown.