The Original Factory Shop has completed a 100,000 sq ft extension to its office and warehouse headquarters in Burnley, taking it to 250,000 sq ft.
Barnfield Construction built the additional space and will retain the building as landlord of the retailer's base in Orient Business Park in Burnley.
The Original Factory Shop is a national UK non-food convenience retailer, with 170 stores and 2,600 staff. Turnover in 2010 was £132m.
Tim Webber, chief executive and managing director of Barnfield Construction, said: "Barnfield view our relationship with the Original Factory Shop to be a continuing one and as such we will retain ownership of the buildings and become a long-term partner and landlord of the Original Factory Shop."
Angela Spindler, chief executive of the Original Factory Shop, said: "The increased office space will help us to execute our ambitious expansion plans which will see new stores open in town centres and communities across the UK throughout the next few years."