Add on or not
 

NFHS A88 (eligible reciever) is held by B15 as he moves downfield, before the pass is thrown. The pass is complete to (a)A88 or (b) A81 either 20 yards downfield or in the endzone for a TD.

Does the penalty "carry over" to the PAT or KO, or the end of the run if not a TD?
Or does it have to be declined to get the results of the play?

Thanks

If the result of the play is a TD, then yes, the foul by the defense can carryover to the TRY or KO. If the result of the play is anything else, then the offense has a choice of enforcing from the previous spot (since the foul occurred during a loose ball play) or they can decline the foul and take the result of the play.

If the defense commits a holding foul and the QB decides to run with it, then you can enforce it from the end of the run (basic spot for fouls by the defense during running plays is the end of the run).

End of the run? No option to enforce from end of run.

Scoring play? Yes, scoring team has option to enforce on try or KO.

Mike- see 8-2, 8-3, 8-4 regarding fouls on scoring plays.

NCAA: no carry over.

If defensive holding occurs, but no pass is thrown, where else but the end of the run would the penalty be assessed from?

I won't presume to speak for maven but I'm fairly certain he was talking about the plays in question, which are both loose ball plays.

By all means, presume. Of course I was addressing the OP.

What if the foul on the defense is a personal foul facemask on the defensive end rushing the passer (he pulls the mask of the tackle blocking him). The QB throws a complete pass for a gain of twenty yards or a TD pass.
Holding is the same as the facemask penalty? Does the personal foul change anything/

Again thanks

Mike, the foul by the opponent of the scoring team is going to carry over. A live ball foul with no score isn't. Changing the type of foul and continuing to ask "what if" isn't going to change the answer. I posted the Rule 8 articles that should answer your questions.