“Problem is that house does have an old septic system with a holding tank only; no drain lines due to rocky terrain.”
You have no leach lines coming out of the septic tank? I see a real possibility that the health department will require you to add leach lines as well!
Subdividing usually requires that you meet all requirements of many departments… sewer, road access, and water availability. There’s no grandfathering that I know of.
If it were me, I’d be talking with septic installers and making plans to put in all of the proper lines, into and out of the tank. If you can’t put the new leach lines on your one acre, perhaps you can get an easement to put them across part of the 23 acres. In the long run, you’ll be glad you did it. If you ever sell your home, you’ll want your septic done right so that it doesn’t cause buyers to turn away.