Should I tell a customer the previous work was bad?

Tell them what the condition is and what it means, specifically and without editorializing. Customers need the facts to make a decision; they rarely need an opinion about the last contractor.

What if the previous work is genuinely unsafe?

Say so plainly and directly, in writing. Structural, electrical, gas, and fire safety issues need clear language and documentation, and unsafe conditions may require a licensed specialist or an inspection.

How do I price a repair when I do not know what is behind the wall?

Price the investigation separately from the repair, or give a range with the specific unknowns named. Committing to a fixed number for hidden conditions is how repair jobs go wrong for both sides.

Fixing Someone Else's Work Without Contempt

Open enough walls and you will find things that make you stop for a second. A structural member notched almost through. A drain with no slope. A repair done with whatever was in the truck that day.

The instinct is to have an opinion out loud. That instinct is worth managing, and not only for politeness reasons.

Why the work is usually bad

Very little bad work comes from someone who did not care at all. The common causes are more boring.

Budget is the big one. The homeowner had a number, the number did not cover doing it correctly, and somebody made it work within the constraint. Frequently the same homeowner who is now standing next to you asking who would do such a thing.

Era is the second. Standards change. Wiring methods, framing details, flashing practice, and material choices that were entirely normal thirty years ago read as wrong now. That is not incompetence, it is time passing.

Then there is genuine unknown. Someone repaired the symptom because nobody found the cause. And yes, sometimes it is just bad work by somebody who should not have been doing it.

You usually cannot tell which one you are looking at, which is a good reason to describe the condition rather than the person.

The language that works

There is a version of this that gives the customer everything they need without a single insult.

Wrong: "Whoever did this had no idea what they were doing."

Better: "This joist is notched deeper than it should be, which is why the floor moves. Correcting it means sistering a new member alongside it, and here is what that involves."

The second version is more useful, more professional, and harder to argue with. It also protects you, because trashing a previous contractor in front of a homeowner has a way of getting repeated, and the trades in any given county are smaller than they look.

Separate the categories

Not everything that is different from your method is wrong. Sorting matters:

Unsafe, meaning structural, electrical, gas, or fire risk. Say it plainly, put it in writing, and do not soften it.
Failing, meaning it works now but will not for long. Explain the timeline.
Not to current standard, meaning it was acceptable when built. Explain the tradeoff and let the customer decide.
Not how I would do it, meaning nothing. Keep that one to yourself.

Mixing those together turns a repair conversation into a sales pitch, and customers can hear the difference.

Document before you change anything

Photograph what you found before it becomes what you did.

This protects everyone. It shows the customer what they paid for. It answers the question that comes up later about whether the problem was already there. And on any job where an insurance claim, a dispute, or a future sale of the house might surface, having a dated record of the original condition is worth far more than the two minutes it takes.

Pricing hidden conditions

Repair work is unknowable at the estimate stage more often than new work. The honest structure is to price the opening up and investigation separately, then price the repair once you can see it, or to give a range with the specific triggers named.

A fixed price for what is behind a wall is a bet, and the person who wrote it usually loses.

The part that comes back around

Someday someone will open a wall you closed. They will find something you did in a hurry, in bad light, on the last day of a job that had gone sideways, and they will wonder what you were thinking.

The generous read is usually the accurate one: you were doing the best available thing under the constraints you had. Extending that same read to whoever came before you costs nothing and makes you easier to work with, which is most of what a reputation actually is.