Presumably, only part of this plinth is directly underneath the structure above it, so the rest of it requires a roof.

http://www.archdaily.com/779929/beken-woensel-west-bas-terme...
"Via this outside staircase one steps on top of the plinth, which forms a communal roof terrace, offering unexpected urban views. From the plinth’s roof one can enter one of the two hotel rooms. The other room is accessible via a spiral staircase, which is part of the red volume. This sequence reinforces the idea of a plinth and a superstructure as independent elements."

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Note added at 35 mins (2017-06-03 11:38:21 GMT)
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http://www.archdaily.com/579268/32-discarded-guggenheim-prop...
"Visitors enter via a wooden ramp that takes them directly beneath the gallery space. From here they may choose to enter the plinth, which holds restaurants, shops and other public venues, or they can ascend into the galleries themselves."

www.cosmovisions.com/monuSoubassement.htm
Dictionnaire de l'architecture : Soubassement. ... Le soubassement est la partie inférieure d'un édifice, d'une certaine importance en hauteur au-dessus du sol et ...

www.utl-morlaix.org/.../larchitecture-contemporaine-et-les-... nov. 2015 - 1 Guggenheim Museum (1956) New York ... du musée proprement dites se trouvent sous terre, à l'intérieur du soubassement habillé de granit.