Conlon Construction, which is itself based in Preston, will remodel the transport hub.
Keepmoat has been selected to build a new 2,600 m2 Youth Zone that also forms part of the scheme.
Other changes include the creation of new public space outside the bus station on the city centre side, as well as changes to the road layout, the bus station concourse and the car park levels.
The projects within the overall development programme are subject to planning permission being granted.
Preston Bus Station is owned by Lancashire County Council and the proposals have been created by them, working in partnership with its design team. Cassidy + Ashton, another Preston firm, is providing planning services, working alongside New York architect John Puttick for the Youth Zone and Bus Station Concourse, and their own architects for the car park.
The bus station was built in the late 1960s to a design by Building Design Partnership and Ove Arup & Partners. Threatened with demolition at the start of the 21st Century, those who hailed it as a Brutalist classic succeeded in securing its listed status.
Conlon Construction and Keepmoat have worked together previously to rebuild a large area of Lancashire Business Park near Leyland, on behalf of the county council, after it was hit by fire in December 2011.