The network announced that three new episodes of the series will go into production later this year with the show expected to begin in January 2015.
"A special, plus a new series of three episodes -- it's a record-breaking run! Of course, it's far too early to say what's coming, but we're reasonably confident that the very next thing to happen to Sherlock and John, is the very last thing you'd expect..." co-creator Steven Moffat said in a statement.
Co-creator Mark Gatiss added that the new episodes will take "Sherlock and John Watson into deeper and darker water than ever before."
"It's taken a little while to get the dates sorted as none of the boys are exactly sitting back twiddling their thumbs but there was unanimous goodwill to make this work, so we're thrilled that 221b is going to be inhabited again," said executive producer Sue Vertue.
The BBC Once's crime drama is a contemporary adaptation of the Sherlock Holmes detective stories. The show made its three-episode debut in 2010 with equally long seasons following in 2012 and 2014.
There is no word on when the episodes will make its way to the United States via PBS.