So in this one Sherlock is possessive and so as a gift John gets his initials tattooed on his hip. Sherlock finds out while they are staying at Holmes' Manor. Sherlock is possessive in this one. That's all I can remember. It's old as I read it early in my Sherlock fandom. But definitely on AO3.
UPDATE: FOUND! It was indeed The Road Less Traveled by verityburns. Available on AO3:
The Road Less Traveled (56924 words) by
verityburns and fanfiction.net:
The Road Less Traveled Great read! I highly recommend it.