The 8,360 sq m ballroom will replace the East Wing, which Trump ordered demolished in October.
Underneath that building was the Presidential Emergency Operations Center, an emergency bunker for the president.
The demolition drew criticism from lawmakers, historians and preservation advocates, as well as an ongoing lawsuit.
Critics objected to the decision to demolish the old structure without federal reviews or public comment, and they have critiqued the massive new facility on aesthetic and historical grounds.
The National Capital Planning Commission, one of two federal bodies required to sign off on the project, has a vote on the changes scheduled for Thursday.
Trump has said the ballroom, which will accommodate 1,000 people, is a necessary upgrade to the cramped and dated reception facilities at 1600 Pennsylvania Avenue Northwest.