It's probably hardening and technology upgrades. No problem with keeping the WH and its resident safe.
Article said it was due to HVAC, plumbing, wiring. For that cost, they must be having to tear it out to the studs.
I don't have a problem with the money being spent, IF it
needs to be done....$375 MILLION just seems to be an incredibly high cost. They have to be doing a lot more than they are admitting to....and that's fine, if it's for the protection of the WH and the First Family, regardless of who that family is.