Schwerin Palace is the seat of the state parliament of the German state Mecklenburg-Vorpommern. It date backs until as early as the 950s but has been rebuild and renovated multiple times since then, so I'm not sure if it counts. Most of whats left today is from the 16th century and from a big restructuring in the 19th century.

If your letting the 950s count than it beats the Palace of Westminster by around 100 years and should be in contention for the oldest building still it government use.