Yes it's free and you can walk around and take pictures. Sometimes certain rooms will be closed but they will tell you that when you get there. They also offer free tours.
Yes it's free to go inside. Parking in front of the building is metered non metered parking you have to be lucky or willing to walk. food in the building is not cheap and not that good, there are plenty of nice eateries in the neighborhood
Yes!
Yes and yes, but you cannot take pictures in security or in legislators' offices without permission.
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