Listed investment manager Rathbone is taking an additional 4,700 sq ft in the Port of Liverpool Building with landlord Downing.
The deal means that Rathbones now occupy in excess of 55,000 sq ft in the building which is its largest base in the country, housing 330 staff.
Downing acquired the historic Grade II* listed building in 2001 and has spent £15m on a comprehensive refurbishment.
Robin Ellis, senior agency surveyor at Downing, said: "Rathbones is a longstanding, valued tenant. It is a testament to the quality of the building's refurbishment and facilities that such a prestigious firm has chosen to take additional accommodation."
Another of the building's tenants has renewed its lease. Cafe Marmalade, which has been operating in the Port of Liverpool Building for eight years, has signed a new 5 year lease with Downing.
The restoration of the Port of Liverpool Building included repairing and cleaning its Portland stone exterior and restoring its internal Terrazzo flooring and marble wall cladding using identical materials. The project, one of the UK's largest privately-funded refurbishments of a listed building, also incorporated new lifts, upgraded internal and external lighting and new disabled access.