After the release of 28 Weeks Later (2007), producer/writer Danny Boyle stated that a third movie in the series had been mapped out, with the proposed working title '28 Months Later'. However, writer Alex Garland said in 2010 that "when we made '28 Days Later', the rights were frozen between a group of people who no longer talk to one another. And so, the film will never happen unless those people start talking to each other once again. There is no script as far as I'm aware." After little progress over the years, Boyle and Garland finally announced in 2019 that they were both working on the project again, confirming in 2023 that it was ready to enter production. The film was finally greenlit in 2024. Although Boyle gave Garland the choice to direct, the latter indicated that he wanted to focus on writing after having directed Civil War (2024).
Imogen Poots, who played Tammy in 28 Weeks Later (2007), expressed interest in reprising her role in a sequel: "It's such an interesting idea to think about. It's weird. I think about [28 Weeks Later] like a first love because it was really the first foray I had into acting and obviously was lucky enough to be with [..] all of these incredible actors. [..] Yeah, I love that film." Cillian Murphy, the protagonist of the original 28 Days Later (2002), also indicated that he was interested in reprising his role if writers/producers Danny Boyle and Alex Garland would return to the series.
Originally, the follow-up was set to be titled '28 Months Later', continuing with the movie's timeline-set title motif. However, with the third movie languishing in development hell and so much time having passed since this world was last visited in 2007, it was deemed more appropriate to name the film '28 Years Later'.