The Parliament building, a magnificent example of Neo-Gothic architecture (although displaying Renaissance and Baroque characters too), is just over 100 years old. In the 1880's an open tender was held for the design of the Parliament building.
Guided tours of the Parliament are available when the National Assembly is not in session. The tour takes about 45 minutes, and is well worth the price, as it covers the main entrance stairs and hall, one of the lobbies, the old House of Lords and the Hungarian Crown Jewels.
http://visitbudapest.travel/guide/budapest-attractions/budapest-parliament/