A new 50,000 sq ft office – the largest of its kind in the city – is being created at The Mailbox as part of a £50 million project to rejuvenate the landmark building.
The giant office is being created, including an open glass atrium-style office reception, and is expected to be on the market next year.
Theo Holmes from agency CBRE said it would be the largest single-floor office on the market in the city.
He said there would be strong demand for the office as the city was running short of grade A space, with the likes of Two Snowhill filling up and Paradise Circus years away.
The office forms part of the current programme of remodelling at the complex, scheduled to finish in 2015, including the opening of an Everyman Cinema in late 2014, a sky-lit roof and expanded 45,000 sq ft Harvey Nichols signature store.
Mr Holmes said: "This is going to be something unique for a city centre location. We did a check of everything available in the Midlands and there was nowhere with a floorplate of this size, especially not in a city centre. The floor-to-ceiling space also means that you can put a mezzanine floor in."
The Mailbox is also home to BBC Birmingham, Network Rail and Associated Architects, among other long-term tenants, and houses Emporio Armani, Nicky Clarke, a Malmaison hotel and apartments.
"We are hoping that in effect we will plug the grade A gap," Mr Holmes added.
"A lot of people are talking about a tipping point at the moment, and there isn't much space on the horizon, so with the likes of Two Snowhill and Colmore Plaza likely to go in the next 12 to 18 months, we can fill that gap.
"We might be the only show in town with that sort of space before the next wave of development heading for 2017 and beyond."
The Post revealed last year the Mailbox was set to undergo work to boost its status as a major mixed-use building in the city centre amid investment in the likes of Grand Central Station and HS2.
It came after the complex was bought by a joint venture between Brockton Capital and Milligan in 2011.
Simon Samuels, of Brockton Capital, said: "Having an attractive office environment set amongst quality amenities is instrumental these days in attracting and retaining staff.
"Mailbox creates a great sense of community and, through the progressive enhancement of the built environment, Brockton is fully committed to making that community stronger than ever."