I have a 400 amp meter base and 2oo amp service panel. The house is 7 years old and it was only a few hundred more for the meter base so I went for it. My question is; below the meter is a box with a 200 Amp disconnect and room to add additional breakers. Can I add a sub panel in my barn connected to a 100 amp breaker in this box?

You may have a 400 amp meter socket, need to be sure you have a 400 Amp incoming service service the meter and disconnects.

If so, then sure, and the 100 Amp feeder for the barn.

Thank you for the response Tkrussell. Today I talked with my electric company and found that, although I have a 400 amp meter base, I only have a 200 amp meter. Can I still add a breaker to this box for the sub-panel I would like in my barn?

Depends on the existing load, and if the 200 Amp service can handle the additional load.

Need to have a service rating load calculation done to confirm.