Wealth manager Rathbone Brothers has taken more space at the Port of Liverpool Building.
The FTSE 250 firm already occupies 55,000 sq ft and has added 10,299 sq ft under a new long-term lease.
It is now its largest base in the country, housing 380 staff.
Rathbones has expanded into three suites on the third floor and taken on the building's boardroom on the second floor, pictured, which will still be available to other occupiers.
Last year Downing completed a £15m restoration and refurbishment project at the waterfront building, which is one of Liverpool's 'Three Graces'.
Robin Ellis, senior agency surveyor at Downing, said: "Since acquiring The Port of Liverpool Building in 2001 we have an on-going commitment to carry out refurbishments that provide the highest quality office space, whilst enhancing the character of the building.
"Rathbones is a highly valued tenant and we are pleased that they have chosen to take advantage of the facilities we provide by taking additional space – we are also thrilled to see the magnificent boardroom back in action."
Paul Chavasse, director at Rathbones, said: "The Port of Liverpool Building is already our largest office but this extension of our lease gives us the impressive historic boardroom as well as improved security and connectivity with our own private lift lobby on the third floor.
"We were keen to stay in the building as it has unrivalled character and is in a fantastic location for us, and we're pleased that Downing was so flexible in regearing our lease."
The building's agents are CBRE and Keppie Massie but the deal was agreed directly with Downing. The length of the lease and the rental values were not disclosed.