The section will grow in size to 5,750 sqm and will be longer than the pitch at Wembley Stadium when the revamp is completed in 2018. The new hall will be large enough to stock more than 70,000 items.
The marble-floored space has been designed in collaboration with architect David Chipperfield and will open in three phases, The Guardian reports.
Selfridges said the project reflected the original ambition of founder Henry Gordon Selfridge to have a unified block in the building.
Chipperfield said: "Inspired by the timeless classicism and grand scale of the original building, the scheme will unify the accumulated building structures that make up the department store.
"This singular identity is further underlined by the introduction of a series of screens that unite the brand concessions."