The build will see construction of an extra 2,000 seats in the "arena bowl" area, bringing the total capacity to 14,000.
Rockers AC/DC were the last band to play the arena in its current format in April and work has now begun on catering facilities, seating and toilets, as well as improvements to backstage areas.
, and will run in parallel to that on the main arena. New hospitality bars, restaurants and a party deck are near completion ahead of a relaunch in October.
"The new arena bowl will be in the shape of a horse shoe with brand new seating in a design focus that will create the ultimate entertainment experience for everyone," said The NEC Group MD of arenas Phil Mead.
Birmingham City Council provided a loan to The NEC Group of £25million for the redevelopment, and Regional Development Agency Advantage West Midlands contributed £4million.
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