The short answer
A typical thin vertical hairline crack is not a dealbreaker, it's common, minor, and cheap to seal. The cracks that deserve real caution are horizontal cracks, inward-bowing walls, and stair-step cracks through block or brick, which can mean the ground is pushing on the foundation. Read the full guide to foundation cracks.
Which cracks are fine
Thin, vertical, hairline cracks in a poured-concrete wall are usually just shrinkage as the concrete cured. They may let in a little water, but they're not structural. Sealing runs about $500 to $2,000. These almost never change the decision to buy.
Which cracks mean 'get an engineer'
Treat these as a stop-and-check: horizontal cracks, walls bowing inward, stair-step cracks in block or brick, cracks wider than about a quarter inch, or cracks paired with sticking doors and sloping floors. These can indicate active movement, and real structural repair runs $5,000 to $40,000+. The move here isn't to walk. It's to get a structural engineer (not a waterproofing salesperson) to assess it before you decide.
When it's an actual dealbreaker
It tips into dealbreaker territory only when an engineer confirms significant ongoing movement, the repair cost exceeds what you can negotiate or afford, or the seller won't allow the assessment. Short of that, it's a number, often a good source of leverage, since visible cracks scare off other buyers.
How to handle it in your offer
- For anything beyond a thin vertical crack, make a structural engineer's assessment a condition.
- Get the repair quoted in writing.
- Use the quote to negotiate price or a seller repair.
- Don't rely on a basement-waterproofing company to tell you whether a crack is structural, that's the engineer's call.