So how is that being extended to an office? (Thanks for the answer btw)
In particular, many of the documents in his office are off-limits to anyone outside of Congress or Congressional staff.
The separation of powers implications any time any branch interferes directly with another make for legitimate concern, but, if you read the Byron York piece at National Review Online, you'll find that the Justice Department lawyers were very careful to insure that privileged documents were not among the things seized. Executed under a proper warrant and with the special protections cited in the article, it's highly unlikely that any of this material would be ruled inadmissible by a Federal court.
Congress's sole redress in that case is to call the AG and some of his subordinates to Capitol Hill for a nasty grilling, withhold budget money for the Justice Department (yeah, sure) or to impeach, convict and remove the AG or the President, which obviously they aren't going to do. So, in defense of the separation of powers they're making some noise (they should) and pretty soon they're going to shut-up (they should do that, too.)