My university's department had a rule for it but I don't think it was ever really enforced. It only seemed to be only used at the discretion of the tutor marking the work, as it wasn't consistent. I recall in my second year I was ~600 words over a count, and I had a small % removed, something like 5% (I can't remember now, I'd have to go find the feedback). In my dissertation I almost doubled the count and I wasn't punished.
Generally tutors told me (particularly my supervisor) would tell me if the added content adds value then you shouldn't worry.