I always have multigenerational-multifamily homes, and every neighborhood invariably has a lot that catches unattached sims, or those outside the neighborhood story. My current neighborhood employs leftovers as servants; they do the cooking, weeding, etc. My old Pleasantview had a lot that collected recent college grads who weren't starting proper households. It was sort of an arrested development house, because the sims there kept living like college students even though some of them had the responsibility of a job.

I had a "Boarding House" for newly adult sims who hadn't told me what they were going to be yet. In hopes that by living together they would tell me.

I don't mind having sims live alone. In my Pleasantview Tara Kat will probably never get married. She has her cats to keep her company and she's perfectly happy. She's also a good witch. I always look forward to playing her. Florence Delarosa also still lives alone. I'm not sure if she will stay single, but it doesn't matter to me either way. I enjoy playing her and her flower shop. Her business is doing well and she meets new people every day. It's because of her that I got to really like OFB.

My single sims are usually pleasure or romance sims. Sometimes they live alone, sometimes with another single sim. They like to be sportsstars, superstars, rockstars, or celebity chiefs. They usually live downtown. Sometimes they are roommates and some time I will put them in a house I have made into two apartments. ( I do not have Apartment Life, but I do have Open For Business and can lock doors. ) When one is nearing the end I like to move another just starting out in with them to inherit. I had two pleasure sims who were first cousins, both of whom are now deceased, sharing a house which was left to the niece of the longest surviver.

In my Meyers legacy (which I should really get around to playing again, but sadly I'm in the middle of moving) I have a founding family that is kind of odd. I don't really like to force my hand in most situations, and I ALWAYS play my way through glitches and embrace them. My neighbourhood is running over three years old and there is no sign of it imploding. A lot of my stories are from random dice rolls or just flipping to a page in my "big book of starter plots" and using whatever my finger lands on.

Basically, it's a soap opera. Prepare yourself.

Claire Meyers is adopted. Her mother was a notorious woman of the night who was paid $600 (a lot more money in the 1960's) to give birth at their house and let Keith and Chandra care for her baby. I did this because after weeks of trying, Chandra still didn't get pregnant, so I thought that she was infertile (this has happened to me before). Claire, to this day, is in her 40's and doesn't know that she's adopted. Claire lost her childhood friend and current lover Julia in a car crash during her university years. Becoming depressed, she dropped out and, on a vacation that Keith paid for, ended up having a drunken one-night-stand and came home pregnant with Cadrie Hutchins-Meyers. Cadrie, during toddlerhood, just simply would not learn to walk. I decided to take that as he couldn't walk at all, and thus he now uses a wheelchair and stairlift. He can't walk more than a few steps without his entire queue dropping, but I'm okay with that, as it makes for interesting storytelling. A few years into Cadrie being a toddler, Claire met Germanium Oleander (NOT GERANIUM, she's named after a beautifully shiny metal and a plant, not two plants - her parents were scientists, one who worked with metals, the other with plant biology), and the two got married shortly after. By anonymous donation, gene stripping, and high science (as this was now 2020 in my game) they ended up both having babies a the same time, Joss and Jamie, who were biologically pretty much theirs. Claire, though adopted, is my legacy's heir. Or, she is until Alecia is old enough, as Jack and Jeff have expressed NO interest in staying in the house they grew up in.

Chandra got the fertility treatment reward from Freetime and ended up having twins:

Jeff Meyers was going to marry a girl that he had knocked up as a teenager, Jessica, but she was kidnapped and ended up having the baby, Sonnerie, in the basement of an insane man's house who was actually Mr. Jefferson, Claire's real father. They had to also raise Alexander Jefferson, his other toddler son, who is also Claire's biological brother (crazy, right). Going to college and moving on, he got with a beautiful woman named Michelle, who also ended up getting pregnant, but she convinced him that she got rid of it and left. She ended up giving the twins to an orphanage. I've lost track of one of them (my town is HUGE), but Mia went to a loving suburban family which Michelle visits regularly. Jeff ended up hastily marrying Laci and adopting a little girl named Gabriella. They also had two biological children, Rosie and Antoinette, but Jeff was put in jail for four seasons just before Antoinette was born.

Jack Meyers ended up marrying one of the survivors of Mr. Jefferson, Natalie, and he also took in Alexander as a foster child. Jessica and Sonnerie didn't make it out. They bought the house next to the legacy lot and had Tyrelle. Then, after fertility treatment after troubles conceiving a second child, they had quads, of which one didn't survive. The remaining three were Avondre, Michelle (accidental double naming), and Neveah. Eventually their house got too crowded, and they decided to up and move to a new modern-styled gated community, with room for everybody.

A few years after Jack and Jeff were born, Chandra had Felicia, who died in toddlerhood. I have mods installed that allow child-and-lower death just to make it more realistic.

And then, in Chandra's elder years, by a glitch, she had Alecia Meyers. Chandra died straight after, and Keith took advantage of assisted suicide and decided it was his time to go, too. Alecia is being raised by Claire and her wife and is doing great.

So, my answer as to what do do with your spares? USE THEM. Make stories of their own. Just because they're spares doesn't mean that they should be left out or ignored. Let them loose on the world.

oooh I put them in one apartment/household usually and they can be super fun. once two ladies lived in an apartment complex and unbeknownst to them they had the same lover so when he came over one night and woo-hoo'd with one the other got furious and marched in and slapped them both. I have one sim that lives alone but she also is a romance sim and has lots of dates, and throws lots of parties, and has friends over all the time for lunch and music.

I also suggest to make alone sims fun, to have them on a farm and work on gardening, or take up animal breeding or even have them set out animal food and see how many strays they are able to adopt that way. They don't always need a family or a spouse to be fun to play ^^_^

I tend to put a bunch of them together as roomies. Since I enjoy playing roomie scenarios, it gives me an incentive to go back and play them some day.

I always try to find a way to incorporate my leftover sims into the neighborhood. I'll take their personalities and interests into consideration, then I put them where I think they'll fit best. At first I dread playing with so many sims, but I end up enjoying where their stories lead.

I make them into dormies at uni. If a playable close in age falls for them, then they become playable again. If another dormie or townie falls for them, they become a townie couple. They may have a townie kid after an appropriate amount of time, thus keeping the children fresh and genes passed down without having to play through everyone. Eventually they will be aged to elder when their playable age cohort does. Even further along, they will be killed off due to old age when a close friend in their cohort passes on.

Age transitions of my playables triggers alignment of their townie friends and relatives.

I rarely have "leftovers" as I usually find something for them to do or acr gets involved & they fall in love on their own. In one hood where heirs mattered more than extra siblings, & the royal heir became an enemy of a sim (die roll when they appeared on a comm lot), they were sent to live in an asylum, with either random, oddly dressed (to me) townies & other enemies of the crown. Depending on a die roll & relationships of families affected, the leftover can either have his/her freedom bought back from the crown or they had to survive the asylum challenge to rejoin society.

Sims that I don't want to play as in 'get them married and breed' either get moved in together until I have enough for an asylum challenge, they join a monastery or nunnery and I play them together until they die. If they are still boring I age cheat them to be where their should be. Some might be sent to a retirement home on turning elder.

Sometimes I group several sims together as roommates where they can start out on their careers and experience life before getting married and having kids. Once my elders start getting old I often place them all together in a nursing home and its so much better then having them in the way in their children/grandchildren's homes.

I try not to have too many single sim households, but that doesn't mean that I don't have single sims. My sims pretty much determine their own lives with the help of their wants and ACR. Some will never have a steady enough relationship to marry/move in together. Some of my single sims live in a motel, others own houses and rent a spare room to a friend for a cheaper price than the motel offers. That's mostly the sims in the city. Single sims who do live alone usually do so in the village, where they have their own garden and usually own pets. Single sims are more likely to be business owners, especially of home businesses. Some of the single sims in my village will end up forming a commune, that's all about living close to nature and art. Yes, an art hippy commune. Free love, baby!

Lots of things!

Mostly they live alone unless they seem really lonely, or they can't make enough money to cover their bills alone. A lot of them get pets, some get to adopt children. I also have a lot of single parents where relationships have failed or the parents weren't together to begin with and decided not to get together.

Sometimes I'll give them a project to complete - like a business to run which means they don't have time for family, or to devote their life to a hobby or something or becoming a supernatural. Sometimes they get a special role, like being caretaker at a school, working as live-in staff at the orphanage, that kind of thing.

If they are really, really boring and I don't know what to do with them, I make them into a townie and just age them up at the appropriate time, when their siblings/friends age up. I can always make them playable again later by having someone ask them to move in.

And some of my sims find love later in life.

A smaller household can be less challenging to play, but it's very different, it changes the dynamic. It means that instead of cycling through lots of sims all with their own things and storylines going on, I can focus on one story and make it really detailed. But this is also why I don't have a set rotation which I follow, because sometimes I'm in the mood to focus on one sim, and other times I like the hustle and bustle of lots of children. I keep my families in sync with the lot sync timer, but I don't play each for a set number of days or decide "OK now I've played X house, I need to play Y and then Z"

A lot of my reason for playing is babies/children, I've found, so there is really no such thing as a permanently one-sim household. There will be a baby at some point, or ten. Not all sims marry, but all sims reproduce. I suppose the male sims have it slightly easier than the female, since they don't require a partner to have a baby, just a telescope, but thems the breaks. On the other hand, the aliens don't pay child support and their offspring have to live in Strangetown . . .

Adopt a child

All sims that makes it to adulthood have children in my game. All. I kinda feel like it's a waste of genetical possibilities if they don't.
Not all get married and fit into a bigger storyline, but they each have smaller storylines like cheating or owning a small business or marrying a servo for a few years before meeting "the one". (Ernie Jr. Eckles does all of those things!)

Edit: basically this:

Speaking as an "Unfortunate Leftover" myself, I feel I really ought to contribute to this thread!

I suspect that, just like myself, many of my Sims will never marry or have children. But they will find lots of other things to do. And I do hope they enjoy their lives! Some of them may even become obsessed with playing The Sims on their computer!

I totally agree with AndrewGloria!

But though I do not consider single sims "unfortunate leftovers", I do have some that fit the bill: All those sims created for storytelling-purposes or playtesting/playing challenges. In general I give them makeovers once they´ve fulfilled their purpose and treat them as semi-townies. I do not incorporate these sims into the rotation at that point, just like any other townie they have to make themselves visible first and contribute something to the neighborhood chronicle.
I am happy for everyone who manages to snatch a bit of screentime and get a life.