Financial services firm PwC will occupy the whole of the first building at the £700 million Paradise scheme in a move which could see the firm add 1,000 more staff to its Birmingham team.
In March 2016, the company announced it had agreed a lease on four and half floors of One Chamberlain Square which is currently under construction on the site of the old Central Library.
It is now extending that 20-year deal to take all 150,000 sq ft of the available office space in the eight-storey city centre building which is due for completion in summer 2019.
As well as relocating its existing 1,400-strong team from its current base in Cornwall Street, the extra space has the potential to accommodate an additional 1,000 people as PwC grows across the Midlands where it also has offices in Castle Donington and Milton Keynes.
Midlands regional chairman Matthew Hammond said: "This is a significant move for PwC and one of the largest regional investments we have ever made in the UK.
"Our new place in the heart of Birmingham gives us the opportunity to grow and deepen the skills of existing teams and client relationships across the Midlands.
"PwC is actively expanding its presence across the UK to match the changing footprint of its clients and make the most of new opportunities in fast growing regions like Birmingham and the West Midlands."
Due for completion in 2026, the wider Paradise project will have eight office buildings, a new hotel, units for cafés, restaurants and shops and new public realm as well as a new traffic system around the former Paradise Circus island.
PwC is so far the only company to sign a deal to take space there.
The Paradise project is a joint venture between Birmingham City Council and funding managed by Hermes Investment Management alongside Canada Pension Plan Investment Board.
Argent is the development manager.
Chris Taylor, head of private markets at Hermes Investment Management, added: "Paradise is of pivotal importance to the rejuvenation of Birmingham city centre as a dynamic commercial destination, attracting talent and global capital.
"It is therefore fitting that PwC has made the decision to further expand its business presence at One Chamberlain Square.
"The extensive enhancements we have made to Paradise, together with world-class place making and environmental improvements, will help transform the city centre and create a sense of pride, identity and belonging for the wider citizens of Birmingham."
PwC was advised by CBRE and Squire Patton Boggs with Paradise represented by GVA and Berwin Leighton Paisner.