Due to security, you can’t park in the Capitol Complex unless you have a pass. To get a pass, you need to be a staff member or an elected official.
If you want to visit one of the legislative office buildings, you can take a golf cart from the public area to a location close to your destination. Here’s a quick video of the ride past the U. S. Capitol. (The quality is pretty bad, but remember I was bouncing along in a golf cart!)