Is a "corrosion" advisory serious?

A corrosion advisory means the tester has spotted rust that isn't yet bad enough to fail the car, but is worth keeping an eye on - it hasn't reached the point of being a genuine safety concern.

Where corrosion becomes more serious is around structural points: chassis rails, suspension mounting points, seatbelt anchorages and similar load-bearing areas. Testers pay particularly close attention to corrosion within 30cm of these mounting points, because that's where rust is most likely to actually weaken something that matters. Corrosion that's severe enough in one of these zones is typically a fail, not an advisory.

So an advisory noting corrosion near a mounting point is worth taking more seriously than one noting general surface rust somewhere less structural - it's the early warning for something that could become a genuine fail at the next test if it isn't dealt with.

Source: gov.uk: MOT inspection manual, structural integrity and corrosion