I always thought the expression “safe as houses” meant that houses were safe places. It’s a beautiful idea; like running home with a late-summer thunderstorm huffing down your neck. There’s the house, waiting for you; a barrier from nature, from scrutiny, from other people. Standing on the other side of the glass, watching the sky playfully pummel the earth like a sibling.
But house idioms and their variants, in fact, often signify the opposite of safety and security. If something is a house of cards it is precarious, easily disrupted. If the writing is on the wall we can see the end of something long before it arrives. If we do not throw stones in glass houses, it is because the house is constructed of hypocrisy, readily shattered. All expressions of weakness, of the inevitability of failure.
“Safe as houses” is something closer to “the house always wins.” Instead of a shared structure providing shelter, it means that the person in charge is secure; everyone else should be afraid.