Another thing to recommend (I used to do this all the time - we all KNOW the builder is not going to rip out the stairs, well, actually, I've had it done a few times, but most clients do NOT want that done) ...
Get money from the builder for "insurance".
Typical mortgage is for 30 years, typical life of a house is 75 years (got this from FEMA at a seminar, they use a 75 life span), figure about $100 per year for additional insurance which specifically covers the non-code compliant stair ... that's $3,000 for 30 years and $7,500 for 75 years. Then I tell my client to 'be prepared to pass that money on when you sell'.
Of course, most clients "forget" that last part ... until they sell ... oops.
I've had many, MANY, clients who *did not want* the stairs ripped out, about as many as there are builders who *do not want* to rip them out. so just pass the money around. Worked for me, my client's loved the idea.
Do you think they used that for the extra specific insurance coverage? Are you crazy, of course not.
However, I did have a few who called their insurance company and were told that the stairs would be covered regardless, as long as no changes were made, and some even got that in writing. As long as the insurance company is willing to pay for a claim on a non-compliant stair, and your client is willing to accept a non-compliant stair, not much else we can do.
$7,500 is cheap for a builder to not have to rip out a stair.