So one of the house rules we play with is that at first level you get your constution score + max class HD in HP. Every level after is standard roll+con mod.
I feel like this rewards characters who invest into having really hardy characters and also allows me to do more interesting adventures at early levels.
Now i don't run alot of APs but am considering Rise of the Runelords after our current campaign ends and wanted to know if this houserule would unbalance the encounters?
That is alot of hp at 1st level. Alot of the early encounters will be much less of a threat. Particularly the goblins who only do a few damage with a hit. If a character has twenty something HP 2-4 damage is alot less threatening then if the character has 10-14 hp.
I'm running RotR right now, about 2/3rd of the way through the Skinsaw Murders and if anything, the PCs need very little help (my group is typically 4-5 semi-optimized PCs). We do max HP at first, then roll after that (re-rolling 1s). I just don't think you need to help the PCs out too much. Perhaps deeper into the AP this will change, but so far, this is how things seem to be going in our group.
I think you'll be fine.
For my run on Runelords, we used the "Flat 6" bonus HP from the PF Beta test rules. That put us all around the same point you'd be with your house rule here.
It really helps at first level, but players can still fall to an ill-timed crit.
But 5th level, the extra HP is basically irrelevant.
You certainly don't have to adjust APL math in this instance. It allows you to go a little bit rougher on the PCs, which is good fun.
We play with full dice+con mod all levels and for monsters too. Much better because the Hp never reach the damage....