I'm sticking my neck out a bit here, but I can't find relevant references to this term and I have a strong hunch that it's being used in the the sense of "assise de réglage", which refers to the top course of the foundations designed to provide a level base for the building.

"assise
A.− ARCHIT. Rang de pierres de taille, de moellons ou de briques disposés horizontalement sur le sol même ou sur un rang inférieur, pour élever un mur ou constituer les points d'appui d'un édifice."
http://www.cnrtl.fr/definition/assise

Here's an Egyptian reference (referring to Karnak) using this term:
"La dernière assise de fondation en grès vert visible constitue en effet l'assise de réglage des murs."
https://books.google.es/books?id=VNUvmGc9e2oC&pg=PA445&lpg=P...

"Assise de réglage" is a term commonly applied to the euthynteria of a Greek temple:

"Dimensions de l'assise de réglage (euthyntéria) : 20.55 m x 47.80 m."
https://books.google.es/books?id=vJHMEuy4KisC&pg=PA16&lpg=PA...

" Euthyntéria
TASY Assise de réglage"
http://archwort.dainst.org/thesaurus/fr/vocab/?tema=40

"Euthyntéria (un) : assise de réglage pour l'horizontalité, sur laquelle est construite le bâtiment."
http://fr.lettresantiques.wikia.com/wiki/Glossaire_d'ar...

And in English it's referred to as a levelling course:

"Levelling Course (Euthynteria)"
https://books.google.es/books?id=-zGwCQAAQBAJ&pg=PR181&lpg=P...

"He shall build the walls of the arsenal and the piers from stone from the Piraeus, having established a euthynteria [a levelling course] for the walls with blocks"
https://books.google.es/books?id=s92KAgAAQBAJ&pg=PA267&lpg=P...


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Note added at 6 hrs (2016-02-28 19:50:03 GMT)
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Here's confirmation that assise and assiette mean the same thing, and that both evidently means "course", which somewhat increases my confidence:

"Assise ou Assiette - Couche de pierres ou briques appareillées dans un mur. - Fondations d'une bâtisse en béton"
https://fr.wikipedia.org/wiki/Glossaire_de_l'architectu...

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Note added at 1 day23 hrs (2016-03-01 12:59:44 GMT) Post-grading
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Great! So that's another one dealt with. It's nice to have confirmation; thanks for letting me know, Hazel.