If you've done your research on querying, you've probably found the phrase "standalone with series potential" is a popular description. For a lot of writers, this can be confusing. So here are some things to know about querying a standalone with series potential.
Standalone novels can be part of a planned series.
To some authors, the term "series potential" sounds like book 2 is a distant possibility. But for many agents and publishers, a book with series potential will automatically turn into a series if the original book sells well. So don't pitch a book this way unless you are really prepared to write a sequel.
Publishers don't usually sign multiple books with a new author.
Because new authors are an unknown factor, signing a whole series is a risky investment. The author might be uncooperative with edits, unwilling to assist in marketing, or incapable of meeting deadlines. The first book is a smaller risk that the publisher can undertake while building a relationship with the author. During production, there is a lot of growing and learning and communicating and establishing patterns. If all goes well, the publisher (and author) has a much better idea of what to expect the next time around. So if book sales are profitable, the publisher will usually be eager to capitalize on the fans, brand, and marketing for book 1 with a sequel. But they won't usually risk a large investment until they've gotten that first book under their belt.
Standalone novels can still set up interest for book 2.
When writing a standalone book, tension should die down after the climax and no new goals should be introduced. The fallout of the climax can be explored a little, but the characters shouldn't make new plans or consider new problems.
Many of the series that you'll have read by big name authors were not standalones with series potential. They were planned to be series from the start and were purposefully designed to make readers beg for the next book. So don't necessarily use popular books as a template. They might lead you astray.
At the end of The Lightning Thief, the real bad guy gets away. But the stolen lightning bolt has been dealt with and the identity of the red bad guy was revealed, so the original questions have been resolved. The immediate danger is gone, but there is still that niggling plot thread to pull readers into the next book.
How is this different from just writing the first book in a series?
In The Hunger Games, Katniss has survived the Games, which was her goal at the beginning of the book. But her method of doing so has brought other problems for her to deal with. She has made new enemies and is now caught up in the world of being a Victor, something she knows little about. Also, Peeta discovers that she was lying to him, creating a new problem for Katniss.
In Harry Potter and the Half-Blood Prince, Harry starts making plans to go on the quest that Dumbledore has set for him. His actions at the end of the book are not focused on the resolution of the recent events, but on the larger goal of the series and his next steps to achieve it. In an established series, this is a good strategy to get readers to come back for the next book.
If you write an ending like The Hunger Games or
Harry Potter and the Half-Blood Prince, you are likely to scare an agent or publisher off with the amount of set up for book 2. Focus on leaving loose threads without the characters acting on them and not introducing any of them after the climax.
Standalone novels lead to more satisfied readers.
Unsatisfied readers of Harry Potter and the Half-Blood Prince may be first in line to buy Harry Potter and the Deathly Hollows, but agents and publishers know that in most cases a sequel is not guaranteed. If Harry Potter and the Deathly Hollows had never come out, the author, the publisher, and the movie makers would all have been in trouble. They may have had to spend the next forty years fielding angry emails, tweets, and other reader contact begging them to address the issues that were left unfinished in the last book. Certainly many fans would not be interested spending money on Fantastic Beasts or The Curse Child if they weren't given the Deathly Hallows first.
But sometimes book 2 isn't possible. Markets change from year to year, Covid happens, authors miss deadlines, or book 1 doesn't sell quite enough to justify the cost of making book 2. If any of these things happen, publishers don't want to have burned bridges with all of book 1's readers. They want to leave things resolved enough that readers are satisfied with book 1 by itself.
This satisfaction will usually lead to more recommendations (since many people feel bad about recommending unresolved cliffhangers) and more word-of-mouth marketing. And it keeps a positive relationship with their readers so now they can ask those readers to read another book, one that isn't book 2, and not have lost them as contacts forever.
This can benefit an author as well, since publishing book 2 of a series without the original publisher is often difficult. But publishing an unrelated book is still perfectly possible. Now their readers can happily read whatever they write next if things don't work out to do book 2. And since readers are still loyal to authors, keeping them satisfied is good business strategy. (It also makes for much nicer fan emails, which honestly can be a huge deal for new authors.)
So if you're thinking about querying a novel as a standalone with series potential, hopefully now you understand better what that means and why agents and publishers are asking for it. If your book doesn't fit the requirements, you can choose to edit it, self-publish it, or write another book that is a standalone and then pitch your series later. But you should never query a non-standalone start to a series under the label "standalone." That's a good way to burn bridges too.