What if Snape didn't live at Spinner's End?
I seem to be in the minority opinion on this, but I don't think Snape kept Spinner's End as a residence, or that he used it as such before the summer at the start of Half-Blood Prince. I have a few reasons for this perspective, partly because I don't think it suits Snape's character and interests, and partly for practical reasons which are best summed up by this post from user deathdaydungeon (who I'm not @-ing because they seem to have deleted all their HP posts so I'm not sure they want to be dragged into this, but it's such a good and relevant post I have to reference it):
Snape’s house in Spinner’s End is a traditional two up, two down through terraced house, mired deep in a maze of identical cobbled streets, overlooked by a looming mill chimney, and seemingly – by the 90s – entirely abandoned.
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...these houses, in this state, no longer exist. A large percentage of two up, two down terraces were demolished as part of slum clearance, which should tell you all that you need to know about the state of the houses.
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These were workers houses, often funded by the owners of the mill, and therefore tied – meaning that rent was deducted from your wage before you received it. There were benefits to being in tied accommodation, including being close to work and having a guaranteed landlord – but that was as much benefit to the mill owner as the worker. Seeing great competition, some mill owners invested in their properties to entice workers – but Spinner’s End is not an example of this; Spinner’s End would’ve been regarded as little better than a slum even when fully occupied.
The narrow streets are indicative of when these houses were built, presumably in the late 1800s – cars were not a concern, and the attitude was to build as many houses on as small a piece of land as possible.
By the time the 90s roll around, and we see Narcissa and Bellatrix descend upon the street, Spinner’s End appears to be mostly deserted. With the closure of traditional manual industries, families would be keen to relocate to where work could be found. Estates which hadn’t already been cleared by the 60s would find themselves left to rack and ruin, their former occupants long gone – whether seeking a new life elsewhere, or having died.
The post also talks about how none of these slums exist anymore - those that weren't torn down and rebuilt were modified and modernized. In HBP however, the slum in Cokeworth is still in an abandoned, but not yet revialized or rebuilt state. The only sign of life we see when Narcissa and Bellatrix visit Snape in Spinner's end (aside from the fox Bellatrix kills) is in Snape's house. The slum appears abandoned:
An immense chimney, relic of a disused mill, reared up, shadowy and ominous. There was no sound apart from the whisper of the black water and no sign of life apart from a scrawny fox that had slunk down the bank to nose hopefully at some old fish-and-chip wrappings in the tall grass.
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Side by side they stood looking across the road at the rows and rows of dilapidated brick houses, their windows dull and blind in the darkness.
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Some of the streetlamps were broken; the two women were running between patches of light and deep darkness.
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...they moved deeper into the deserted labyrinth of brick houses. At last Narcissa hurried up a street called Spinner’s End, over which the towering mill chimney seemed to hover like a giant admonitory finger. Her footsteps echoed on the cobbles as she passed boarded and broken windows, until she reached the very last house, where a dim light glimmered through the curtains in a downstairs room.
-Half-Blood Prince, Ch. 2
No one is living in this neighborhood. The author uses the word "deserted" and makes clear that every house is dark and dilapitated, that there's no sign of life anywhere. The broken street lamps indicate that it's more likely the local council forgot to turn the rest off, than that they forgot to fix the broken ones. The fact that the neighborhood is abandoned is important, when you consider that it's unlikely any of the people who once lived there owned their home.
Like the other residents who left, neither Snape nor his family kept the house in Spinner's End, because it was never theirs to keep. In HBP Snape is squatting in his childhood home because he knows there's no one to notice him doing so, though presumably he would use protective spells to ensure no muggles noticed the house was inhabited. The house is described as being in poor shape as well, and as Snape is a pragmatic man, this begs the question: Why is he squatting there in HBP?
My main assumption is that it's the perfect place for him to use to have privacy away from Hogwarts, which he requires for his work as a spy. I see no reason why teachers wouldn't be able to remain at Hogwarts during the summer holidays, and if they can, I also see no reason why Snape would choose to be at Spinner's End for an extended period of time. At Hogwarts he has access to his personal potions stores, facilities, has three square meals provided, and a vast library at his disposal. Given the intellectual curiosity shown by the Prince in his Advanced Potion Making book, these resources would be highly valuable to Snape, especially during the 2 months of the year when he doesn't have educational responsibilities taking up his time or interruptions to worry about.
I can see him being nostalgic or indulging his guilt and occasionally visiting Spinner's End or the surrounding parts of Cokeworth that hold sentimental value to him, though my own reading of his character leans more towards him shutting his emotional self off completely and not allowing himself to indulge in sentimentality that might render him one of the fools who wear their hearts on their sleeves, as per his admonition to Harry during Occlumency lessons. It's a long walk to the Hogwarts gates before you can apparate, and as he's written, I see his self-discipline win out before he makes it there if ever his sentimental side tries to speak up.
If he doesn't spend summers in Spinner's End usually, then what changes by HBP? Mainly that Voldemort has not only returned (which he did a year earlier), but that he was exposed at the Ministry and the wizarding world is now aware of his reappearance. In the process, his most trusted allies have been arrested, including Lucius Malfoy, a key link to giving him backdoor access to Ministry officials and channels of power. The social map of the Death Eaters has drastically changed overnight, and Snape is now at the top of the pack because he has not only proven his loyalty to Voldemort over the past year, but also his usefulness in being close to Dumbledore and having access to Harry Potter. This means Snape needs to cement his position and use it to his advantage, which includes making the best of the short period of time between school years when he's not obligated to remain at Hogwarts.
Since Cokeworth is abandoned, his parents no longer live there either. Presumably when the mill closed, its workers were evicted and forced to move on, whether by the need to work, or the mill's owner who no longer had workers to collect rent from and therefore abandoned their duties as landlord. We don't know if Snape's parents are still alive, but presumably he would have received word from at least one of them that they'd moved and the slums he grew up in were left uninhabited. Because this happened all over the country over several decades, and would have posed a fear for workers reliant on factories for both work and housing, Snape would have known what the factory closing and his parents moving meant, and that the whole neighborhood would be abandoned.
And so when Voldemort is caught at the Ministry, several of his key followers imprisoned, and privacy is more vital than ever, Snape is able to put himself in a position where he can provide just that. We don't know if Voldemort ever visited him there, or how Narcissa knew where to find him. It's possible she had been there many years earlier when Snape was much younger and the neighborhood wasn't abandoned yet, or that he gave her clear directions, should she ever need them, because Snape - or perhaps Dumbledore - anticipated Voldemort's moves and presumed he would punish Lucius through Draco and that inevitably Narcissa would be desperate to protect her son and ask for help. What we do know is that Snape spends some time during that summer at Spinner's End, that Wormtail is assigned to him likely for the purpose of reporting any suspicious behavior to Voldemort, and that this serves as the perfect setting for Snape to prove his loyalty and reliability.
They had stepped directly into a tiny sitting room, which had the feeling of a dark padded cell. The walls were completely covered in books, most of them bound in old black or brown leather; a threadbare sofa, an old armchair and a rickety table stood grouped together in a pool of dim light cast by a candle-filled lamp hung from the ceiling. The place had an air of neglect, as though it were not usually inhabited.
"The place had an air of neglect, as though it were not usually inhabited."
Moments later, Snape points his wand at a wall of books to reveal a hidden staircase - not a feature found in 2 up 2 down terraced houses in factory slums. This indicates that Snape has modified the home and begs the question, what else has he modified? Were the books always there (perhaps Tobias was an intellectual who fell on hard times), or did Snape conjur them to make the place look inhabited? Most of the books are bound in black or brown leather - he could have taken a few existing ones and multiplied them with a simple charm, and perhaps if Narcissa or Bellatrix had bothered to look through they would have found they're the same few volumes over and over again. The sofa is threadbare, the table is rickety, there doesn't seem to be electricity in the house if the candle-filled lamp is any indication - these may have been items left behind, so if it weren't for the books, the place would look abandoned.
With a few simple spells, Snape could have made the place seem inhabited, even if it wasn't. Voldemort doesn't concern himself with muggle affairs, and neither do his followers, so it's doubtful anyone would be aware that Snape didn't own the house. It would have been a convenient place not only for the privacy necessary for his role as a spy, but as both a connection to Voldemort through their shared childhood poverty, and a way of convincing Voldemort (who, for all intents and purposes, operates like a cult leader) that Snape has exploitable vulnerabilities.
It's supposition from metas, but nevertheless likely given Snape's socioeconomic background, isolation at school, and Voldemort's cult-leader approach to controlling his followers, that Snape was drawn into being a Death Eater by his vulnerabilities being exploited. A reminder of his poverty through Spinner's End is a reminder of these vulnerabilities, and gives the impression that outside Hogwarts Snape has next to nothing and is reliant on Voldemort to have options outside of Dumbledore's employ. It's both a brilliant tool to cement Voldemort's trust, and also a great flex - because for Snape to stand in his abandoned hovel of a home and hold enough power for Narcissa Malfoy to beg him for help on her knees is truly impressive. Just as he does when he says to Bellatrix, “You think he is mistaken? Or that I have somehow hoodwinked him? Fooled the Dark Lord, the greatest wizard, the most accomplished Legilimens the world has ever seen?” He's saying one thing, but its meaning is entirely different and known only to himsef. So too, he's saying one thing by presenting Spinner's End as his home outside of Hogwarts, while its meaning becomes different when Narcissa prostrates herself to him and shows that he's the one with power and influence, the working class half-blood from the slums, not her, the upperclass pureblood.
Throughout the books Snape is presented as enigmatic because he's constantly hiding the truth - about himself and about his intentions. What could be more of a mask than claiming Spinner's End as his home? For British readers, it would have been more inherently understood that he could not have owned that home, nor was there anyone left to pay rent on it to. Why would this man, full of ambition, with a home in a prestigious institution, who suppresses his feelings* and eschews sentimentality, spend his 2 months of summer holidays squatting in his childhood home?
It makes more sense to me that when his role as spy required him to spend time away from Hogwarts to enable Voldemort to assess his loyalty and allow for moments like Narcissa visiting him, he thought of the first place he knew that suited - the abandoned factory slum where no one would notice a light on at the end of a dark street, where short-sighted power-hungry wizards would see only vulnerability, and where he could manipulate perceptions of himself on his home turf.
*Unless he's triggered, and it's key to his character that those are the only moments in which he expresses emotion unless he's alone with Dumbledore.