A reminder regarding 400-amp services ....
There are two types. One is a true 400-amp service, with a current transformer. The other is really a 320-amp service{ that is, a 320-amp meter base that can handle surges to 400 amps.
If your load calculations exceed 320 amps, you get to put in the more expensive CT-type service.
Talk to the PoCo; Our OP might be able to simply install a trough with provision to add more meters / services to serve the future additions.
