If there is no water expected, design for water 1 m above bottom of structure, or 1/3 of depth below ground surface, which ever is worse.
If there is any water expected, design for water 1/4 of depth of structure below ground surface.
Water may come from burst mains, climatic change etc., so design for the "water expected" situation, or provide external drainage system. Alternatively advise the client against having a basement.