Science Fiction is a "big tent" genre, and we all know it when we see it. Even if we argue about the specific tropes – and what might make something lean heavily towards another classification (science-fantasy, speculative fiction, etc), is every story that takes place in the future "science fiction?"

Are there exceptions?

Books set in the future are Speculative Fiction

Speculative fiction is an umbrella genre encompassing fiction with
certain elements that do not exist in the real world, often in the
context of supernatural, futuristic or other imaginative themes. This
includes, but is not limited to, science fiction, fantasy, superhero
fiction, horror, utopian and dystopian fiction, fairytale fantasy,
supernatural fiction as well as combinations thereof (e.g. science
fantasy).

A large portion of speculative fiction works are science fiction. But they can be other sub-genres instead. All science fiction is speculative fiction.

While speculative fiction can be set in any era, if the story is set in the future, that makes it speculative.

To be science fiction, the story must depend upon fictional science or scientific achievements. Although that is most plausible for the future, it could be set in the present or past; e.g. we could push some modern, actual scientific achievement back into pre-history; e.g. the Chinese invented gunpowder in 700 AD, it took them 200 years to realize it could be weaponized. They also invented quench-hardened steel, about 400 AD, and had a weaker form of steel around 200 BC. So what if some genius had invented actual rifles accurately firing bullets about 700 AD? That story would be science fiction, set in the distant past.

A story simply set in the future, where the plot is not driven by scientific developments, even if the story uses plausible scientific developments; would be speculative fiction, not science fiction.

The exactly Yes or No answer to this will vary depending on which
school of thought you filter the question through, and how it defines
'Science Fiction':

If we look at it from a viewpoint leaning towards one far end of the range we can find a Sci-Fi definition that limits things to fiction that explores the use and impact of advancing technology on characters/society. This school of thought revolves around the idea that the story heavily relies on "X exists, therefore there is an impact of Y..."

• If food replicators exist, the story will explore how that has impacted society?


• If functional AI exists, how does that interact with humans and what are the impacts to society?


• How are human lives and relationships changed if automation replaces all employment?

From this school of thought, something like starwars is NOT really part of
"Science Fiction", as while it heavily features "advanced sciency
looking things", the story itself does not actively explore technology
or its impacts - The same story could readily be reskinned to exist on
ancient earth without really changing 'the heart' of the story, in the
same way that Shakespearean plays are readily reskinned into a modern
day setting.

Under a stricter school of thought like this, the answer is very much a "No, not every story set in the future is Science Fiction", and a cop drama set 1000 years in the future with flying cars is still 'just a cop drama' if nothing beyond "Props and costumes" would change if you retold it set in the 1950's.

Then at the other far end of things you get a school of thought that says "Anything vaguely futuristic is Sci-Fi", and is often subscribed to by the same people who unhelpfully label bookstore shelves "Sci-Fi & Fantasy"...

From this point of view there are effectively no differences in
classification drawn between Starwars and Star Trek, while in the
first school of thought may find that even specific
episodes/storylines of Star Trek fall short of a 'true' Science
Fiction classification.

The TL:DR is basically that "Art is subjective, and definitions can be flexible".

No.

Science fiction is defined as

a form of fiction that draws imaginatively on scientific knowledge and speculation in its plot, setting, theme, etc.

source

A story could take place in any time period, and meet that criterion, after all, Star Wars, one of the most popular Sci-Fi works of all time, is stated to have taken place 'a long time ago'. Along with this, a book can take place in the future and not be Sci-Fi, though if it does then it's more likely in the near future. A book meant to take place a year from now might not have many new technologies, a book meant to take place in 1000 years probably will.

As other answers have said, it depends on the story.

Another frequently-used genre is Future History. There are many examples of this, varying from the dry-as-dust textbook to complete classic. As a category, it can be hard to distinguish what's Future History and what's simply a novel set in the future. Generally though, a series of novels or short stories/novellas following a future society over a generation or more can be considered to be Future History, in addition to any other category they may fall into.

Future History does require the time period to be in the future relative to ourselves, or perhaps for our own planet to be unrelated to the story so that the time period relative to ourselves is immaterial. Where the time period is in the past and changes facts we know about the past, or where the time period may be in the present or future but with past events changed, we are instead dealing with Alternate History.

As time progresses, fiction set in the future will inevitably be overtaken by elapsed time. 1984, 2001 and 2010 have all come and gone. This does not generally change the categories for fiction set in the future at the time of writing - instead it is viewed as a window onto social attitudes at the time of writing, by what changes the author expects to occur over that time period.

If the author continues writing as elapsed time overtakes him, the result may be an Alternate History with a recognisable divergent point. Alternatively (as with Tom Clancy) the Alternate History may end up being some combination of real-world events and previous events in the Alternate History timeline. (In general it doesn't pay to try to look too closely at how the two tie up in that case, because you can only fit so many Presidential elections and other events into that timespan.)

Alternate History can be past, present or future, of course, which means this is not restricted to science fiction. There is an entire sub-genre of Alternate History considering "what if the Nazis won WWWII?", with novels set in varying eras. Other popular Alternate Histories from the past consider "what if the Roman Empire never fell?" or "what if the South won the American Civil War?" Tom Clancy is an example of Alternate History in the present. Near-future Alternate History might include Games Workshop's Dark Future setting, or Neal Stephenson's Snow Crash. Neal Charles Stross's later series of Merchant Princes novels (starting with Empire Games) is unusual in being a near-past Alternate History, with a divergence point in the mid-2000s where a nuclear attack from a parallel universe puts the USA onto a total-war footing; the resulting novels are set slightly in our past, but the technology level is significantly different from the present day.

It is also possible for writers to go the other way, which generally comes as a plot twist. For one example of this, Battlestar Galactica appears to be Future History until the final season, but turns out to be Alternate History instead.