Civil engineering contractor Mott Macdonald Bentley has signed a 10-year lease on the former Carr Hall Garden Centre in Langho near Blackburn to create a 40,000 sq ft office.
There will be 300 staff based at the office to service a major contract with water company United Utilties, which will partly occupy the new premises.
The former garden centre, owned by Andrew Donelan, who ceased trading there in January 2014, is now being refurbished by the new tenants.
The agent advising Donelan was Alex Taylor at Taylor Weaver Chartered Surveyors in Blackburn, who said this is the largest office deal in East Lancashire for several years.
Mott MacDonald acquired long-standing joint venture partner JN Bentley, based in Skipton, in September 2014.
The garden centre was marketed with a quoting rent of £6/sq ft.
No one was available at Mott MacDonald Bentley.
Trevor Dawson Chartered Surveyors was the joint letting agent with Taylor Weaver.