Are ex cathedra papal pronouncements such as the Assumption of Mary on the same level as the New Testament? I.e. should such teachings be considered part of the New Testament e.g. as an extension to it or similar?
They are considered infallible interpretations of scripture and apostolic tradition, not new scripture itself.
OK, thanks. Is it considered on the same level as Scripture?
Both Scripture and Tradition are considered to be equally valid sources of revelation.
I want to be careful here, as there is no new revelation in any papal pronouncement. In that sense, it is beneath scripture. Various ecumenical councils offered infallible infallible interpretations of scripture (the divinity of Christ, the divinity of the Holy Spirit, the two natures of Christ in one person, that Mary is the Mother of God (this is a Christological statement), etc . . . These councils did not declare new scripture, but they did offer infallible interpretations of scripture such that anyone who disagreed with these points was known to be in error.
So . . . They are not new revelation. They are not the same as scripture. They are second to scripture. But they are still binding, infallible statements regarding what was revealed in scripture, and they make sense not as independent statements but as being entirely dependent on God’s revelation in Scripture and what was passed down to us from the Apostles in the first place. They are not part of the New Testament or any “Newer” Testament that may be.