don't sign the letter. it looks like that the design of the wall

mentioned in the letter may have potential problems. the architect seems realize it now or someone has already found out the problem and pointed out, so the architect now tries to limit his responsibilities if anything happens related to the wall. if you sign, you release him from any possible law suit, even if it is not direct from you. He is liable for any damages caused by the wall he designed. Since this can be a class action (not a single individual sues him), anyone suffers from such damage can be compensated. But if you sign the letter, you cannot get any compensation since you release him from any liabilities he has.