IMHO I take buffer zones and protection zones already as the part of WHS, because they are also unambiguously assigned in the nomination file and guarded by advisory bodies.
Except that they are not supposed to contain anything of recognised or claimed "OUV" (I have seen AB evaluations making this point) and their boundaries are designed purely to protect the OUV of the core zone rather than the value of anything inside the buffer zone. I wonder how this "policy" is reconciled with extensions INTO buffer zones, thereby presumably claiming OUV which had not previuosly been recognised (unless the OUV case is amended at the same time)... but i am not clear how many "extensions" are of this type. Most choose an extension area in a different place (??)
Agree however that it is common for places around the World to claim "WHS" status for areas beyond the core zone - Colombia's Coffee Cultural Landscape and Spain's Serra de Tramuntana are 2 egregious cases I have come across where signs with UNESCO logos are scattered far and wide beyond even the buffer zones! No need for "extensions" when they can gain the "touristic" benefits of inscription without doing so.
Have had a quick look through the 86 sites with extensions and I think it is true that relatively few consist of extensions into previously defined Buffer Zones.
a. Many are extended into totally separate areas
b. Some extend sites which were inscribed before Buffer zones were (almost) compulsary.
c. Some extensions are accompanied by a change to the OUV criteria or at least their textual descriptions thereby "changing the rules" as to what consitutes the site's OUV
d. Some are extensions into a different country and therefore not into an existing buffer zone.
e. Some are "extensions" of criteria with no change in boundaries
The 2014 Bialowieza extension certainly seems to be one which did extend into previous buffer zones without altering Criteria or OUV (though it did remove a Criterion used by Belarus).
On the other hand I don't hink that the Budapest extension to Andrassy actually went into any previuos Buffer zone .....and, in any case "developed"/changed the Budapest OUV and Criteria