Unable to answer "What are the opening hours here?" – Mairie annexe de Saint-Herblain (Town Hall) – https://osm.org/node/1149024686 via StreetComplete 57.3:

Mairie annexe fermée.

Attached photo(s):
https://westnordost.de/p/211157.jpg
https://westnordost.de/p/211158.jpg