The Corporation Road development will include a new parts department and workshop as well as a van service centre on site to ensure specialist fleet advice is available to commercial vehicle customers.
There are currently 12 bays to repair commercial vehicles and 15 bays to repair customers' private cars. A total of 13 more will be created, as well as the two new forecourts for used cars.
Planning permission has been granted to create a new Best Price Car Supermarket at the front of the Corporation Road, Grimsby site, for cars aged between about six months and three years.
There will also be a "Trade To Go" showroom at the side of the eight-acre site.
The vehicles on sale there will be aged between three and 10 years. This will be in partnership with Rapid Fit, which will work on the cars before they go back on sale.
Construction work is currently underway, so Hartwell is currently operating out of temporary buildings on site.
The building work is due to be completed by August with an official opening scheduled for September.