In my campaign the planes have certain specific connections that occur under specific circumstances. This is much like the planar alignments and manifest zones that occur in Eberron. When things align properly, creatures can cross back and forth between the planes.
Often these are tied to celestial events (lunar phases, solstices and equinoxes), but some are much more simple than that (Frost giants crossing into Midgard from Jotunheim during a terrible blizzard in a high mountain pass, for example)
I want planar travel to be more difficult, but not impossible, when things are not aligned properly. This is an extension of a house rule I created years ago to explain why demons were imprisoned in Stonehenge-like menhir circles, rather than banished back to their home plane. The caster's effective caster level for spells like Dismissal and Banishment is divided by four. During the proper planar alignment, it was merely reduced to one half its normal value.
So, expanding upon that, now that my players are 12th level, and I'm looking forward to a bit of inter-planar activities, here are my proposed house rules for plane shift, shadow walk (only for trying to reach other planes, not merely using it as a teleportation effect on the material plane), and other such abilities.
Interplanar Travel: Unless you are using the correct "pathways," interplanar travel is impeded. You must make a Spellcraft check (DC 30 + spell level) and a caster level check against a DC 20, but for this check your caster level is cut to one-fourth (round down) of its actual value. Attempting it at certain times of the year can raise your caster level to one-half it's normal value, as can performing it at a proper location. Performing it at a proper location at a certain time will allow it to work without a hindered caster level, or without the need of a Spellcraft or caster level check.
Example: Plane Shifting to Vanaheim under no special circumstances would require a DC 20 caster level check, but the caster's own caster level is reduced to 1/4 it's normal value. Attempting to plane shift from a ring of standing stones (note, most of them don't work for this due to Forbiddance effects around them) requires the same check, but the caster's own caster level is only halved. The same is true of attempting it at a random location on the new moon, or at the vernal or autumnal equinoxes. Combining two such effects (standing stones/fey ruins, new moon, equinox) requires the same caster level check, but with no reduction in caster level. Combining three such effects requires no caster level check; it works automatically, and sometimes a portal opens on its own under such circumstances.
Each plane will have its own rules about "unimpeded avenues" and reduced caster level impedance.
