Client the National Exhibition Centre named Galliford as preferred bidder last September following an initial tender race based on an £18m price.
But both sides were unable to agree a final price and parted company at the start of the year.
At the time The NEC said it had worked hard on the design to ensure that the scheme was affordable for the market and did not want to rely just on market forces to bring the project on budget.
BAM, which bid in the original tender race along with Wates, Sir Robert McAlpine, Skanska and Carillion, took the refurbishment job second time round with a £24m bid.
Revamped NIA with new cladding
The firm will carry out the complex task of improving facilities in Birmingham while the building continues to open its doors as a major entertainment venue.
The biggest change at the 20-year-old venue, part of The NEC Group, will see a glazed frontage overlooking the neighbouring canals, three towering sky needles and steps down to the waterway.
The plan to extensively upgrade facilities with new public concourse levels to increase the food and drink facilities available pre, during and post show. The works will also enhance the area around the arena.
Considerable M&E enabling works will be required throughout the various phasing to ensure that existing services are maintained and integrated with new.