As safe as houses = completely safe.( alludes to the paying character of house property as investment- an expression to  satisfy a doubting person ” Oh it’s as safe as houses”

• I locked your jewellery in a drawer, so it ‘s as safe as houses.
• If you buy treasury bonds, your money will be as safe as houses.

Bring home to someone = to make someone understand something much more clearly than they did before.

• When I saw for myself the damage that had been caused, that really brought home to me the scale of the disaster.
• The crash brought home the danger of drinking and driving.
• The episode has brought home to me the pointlessness of this war.

Hold the key to = to have control of something ; be the decisive factor in.

• Tourism holds the key to the region’s economic recovery.
• This passage holds the key to understanding the entire novel.