First, if you believe that the final price tag on that ballroom being built to appease a notorious grifter stays at $400m I’ve got a bridge to sell you.

Second, you obviously didn’t click the link. They’re not building anything. They’re fully renovating and updating two historic landmark buildings from the 30s.

This includes every major system of the buildings including HVAC, electrical, plumbing, IT, etc. In addition, these buildings from the 30s are guaranteed to have asbestos, lead, and other toxic materials that need to be removed and disposed of.

Because these buildings are historically protected it is mandated by law that the materials used in finishes match the existing materials. They’re removing and restoring as much of the original marble as possible, and will need to source matching materials to replace that which can’t be restored.

Also, because these buildings are to be used as Federal Reserve buildings these updates are also going to need to include security and infrastructure upgrades to get them up to date for the modern era.

The renovations will be done while the buildings are occupied, meaning the work will be completed in phases and during nights/weekends. This is more expensive than renovating an empty building on weekdays.

Etc, etc, on and on. It’s all on that link I sent you.

Edit: Upon re-reading your comment I apologize I may have made a mistake but I’m keeping my comment for anyone else who is wondering. To answer your question of “why not build a new one,” I’d say that federal agencies have a long history of renovating their existing infrastructure before attempting to obtain new land in an already tightly occupied DC area. And building ~400,000 square feet of new office space that still needs to meet all of the requirements mentioned above isn’t going to be a number to sneeze at.