Hello.  We buried our pet cat 5 yrs. ago beside our house.  Now we have plans to build an addition on our house shortly, (in the same spot and an excavator is coming to dig out all the ground) and I have been digging everywhere in the spot so I can find and re-bury the skeleton but I have found nothing.  Is it possible that every bone has been decomposed as well?

yes it is possible that every bone has decomposed after 5 years, especially for small mammals. the process and study of this is Taphonomy

Depending on quite where you live and the local conditions it's perfectly possible. Larger things like the head of the femur might well survive, but it would be easy for you to miss this and it would likely have lost some shape. There's probably little if anything there for you to find.