AU: John manages to get 221B at a discounted enough price to live there on his own... he thinks, until he discovers someone else has been living there all along. Sherlock is tiny enough to fit into the Palm of John's hand and impossibly brilliant, and with his guidance, John winds up helping the police. Sherlock is ecstatic to have relief from his boredom, and interacting with John isn't half as painful as previous attempts to show himself to regular humans.

John/Sherlock preferred, John/Mary and/or John/Mary/Sherlock also possible, gen accepted.

Bonus: Sherlock eventually becomes human-sized (whether he was born that way and became small or was born small).