We have so far identified 25 WHS meeting the rules for this "Connection".
In fact we have identified no rules for it at all, having the belief i guess that the definition was pretty self evident, clear and unambiguous.

How wrong we were! I have just noted the existence of the "World Heritage Earthern Architecture Program" (WHEAP). A report analysing and describing those sites which use "Earth" as a construction material identifies 150 of them!

This inventory contains a few surprises - The walls of the Alhambra for instance are of packed earth. The report itself contains some wonderful photos of the 150 sites - well worth perusing - http://whc.unesco.org/uploads/activities/documents/activity-21-20.pdf

This opens up of course the question of what we should do about our current "Connection list" with its pathetically small number of sites! The report includes a definition of what constitutes what it subtly changes to call "Earthen Heritage" compared with its title of "Earthen Architecture" .
" properties were selected when earth materials were used in:
• Load-bearing walls (different techniques, rammed earth, adobe, cob, hand shaped earth);
• Mortars, in stone or burnt brick walls;
• Fillings for wooden structures, mainly as part of "wattle and daub" constructions, with many variations;
• Roofs and floors, often in conjunction with wooden load-bearing structures;
• Coatings and paints, exterior or interior;
• Extensive landscaping works requiring specific engineering solutions."

The wider definition undoubtedly allows in a number of WHS which might have struggled to get into our connection even if we had known about their detailed construction. However, I see some benefit in adopting the "official" UNESCO definition in much the same way as we just have for "Cultural Landscapes"

Here is the link for the WHEAP which leads to the above report and many other related links
http://whc.unesco.org/en/earthen-architecture/