Is it legal to camp on a non-designated site if you have a PMA permit or does the lake have to be in a PMA? The lake I'm looking at has no campsites and would be considered a "bushwhack" lake.

The PMA permits are for a specific PMA. Are you talking about a bushwhack lake that is outside of a PMA with no campsites? If so, I don’t think there is anyway to legally camp on it.

andym is correct.

If you have a PMA permit, you can camp in any zone within that PMA that you have a permit for on those designated nights. There aren't any specific campsites in PMA areas, as all the fire grates have been removed, but often there are very obvious places (often old camps sites) to set up.

If the lake is outside a PMA in the BWCA proper & does not have a designated campsite, it is illegal to camp there.

Ok I figured that was the case but someone's comment confused me.

The forest service encourages you to not camp in same spots as everyone else in PMA's. Leave no Trace