Are you sure it's not leaking because of a bad gasket under the cap? Is the gasket even there? I had a little issue where the gasket on the 535i was too loose and actually fell out into the can while putting the cap back on ... leaked massive amounts of oil from under it.
As far as the housing itself ... as far as I know there's only one gasket between it and the engine; everything else is one piece. Unless it has somehow developed a crack (really doubt that one), that could be the only other issue -- I don't see why the whole thing should be replaced.