With the recent hubub over Rockstar Games shutting down a GTA5 modification, some uncertanty has crept into the Grand Theft Auto modding scene. Rockstar attempts to allay these fears with a new statement addressing the topic. Basically, they say single-player mods are okay as long as they don't infringe on any IP or impact multiplayer play:

Question: Are PC Single-Player Mods Allowed?

Answer: Rockstar Games believes in reasonable fan creativity, and, in particular, wants creators to showcase their passion for our games. After discussions with Take-Two, Take-Two has agreed that it generally will not take legal action against third-party projects involving Rockstar’s PC games that are single-player, non-commercial, and respect the intellectual property (IP) rights of third parties. This does not apply to (i) multiplayer or online services; (ii) tools, files, libraries, or functions that could be used to impact multiplayer or online services, or (iii) use or importation of other IP (including other Rockstar IP) in the project. This is not a license, and it does not constitute endorsement, approval, or authorization of any third-party project. Take-Two reserves the right to object to any third-party project, or to revise, revoke and/or withdraw this statement at any time in their own discretion. This statement does not constitute a waiver of any rights that Take-Two may have with respect to third-party projects.