Richard Castle Investigations is the name of the business Castle creates in season 7, episode 11, Castle, P.I.. Having been banned from the 12th Precinct, Castle gets licensed as a private investigator and opens a modest office in a building with vintage design. His rationale is that in this way, he and Beckett might still be able to work the same cases despite his banishment.
In season 7, RCI is featured in Castle, P.I., Private Eye Caramba!, and I, Witness; he becomes discouraged after the death of a former schoolmate, which he was unable to prevent, but decides to keep the office open. After the events of Resurrection and Reckoning, Castle is un-banned from the precinct.
At the beginning of season 8, Castle tells Beckett that he needs to work some cases as a P.I. in order to write the business off on his taxes. When he goes there, after an apparently lengthy absence for its remodel, he finds Alexis has taken over and solved four cases already, in addition to supervising the remodel.
The new RCI office is more lavish and modern than the original design. Its features include videoconferencing technology, a spring-loaded handgun hidden in the desk, and a secret room that can be opened by pulling out The Complete Works of Edgar Allan Poe on the bookcase behind the desk.
The secret room, which Castle exclaims "turned out so much cooler than I thought it would!", includes a fully stocked bar, satellite TV, and a couch that Castle uses at least once for a bed.
The office location is featured in several season 8 episodes, and Castle's P.I. status is a plot point as well.