(as|) safe as houses

not causing or involving any danger at all

The idiom "safe as houses" has uncertain origins but is believed to have developed from the perception that houses are stable, secure, and reliable structures, symbolizing safety and protection. In its modern usage, it is used to describe something or someone that is extremely safe and secure, often emphasizing a high level of confidence in the safety of a situation or investment.

With the state-of-the-art security system, your valuables will be as safe as houses in our vault.
After getting the all-clear from the doctor, she felt safe as houses about her health.
The experienced pilot made the turbulent flight feel as safe as houses for the passengers.
Investing in government bonds is considered safe as houses because they are backed by the full faith and credit of the government.
In this small, tight-knit community, leaving your door unlocked at night is as safe as houses; everyone knows and trusts each other.