(as) safe as houses: completely safe and secure; (in early use also) certainly, undoubtedly.
Subcategories:
— there is no danger (1)
— in safety or security (3)
— within the region where one is safe (1)
— in a secure place (2)
— beyond danger of something (3)
— out of the way of (doing) harm (1)
— not likely to be injured by wetting (1)