Familiars are spirits that can be sent to a pocket dimension indefinitely.  This suggests that they don't need food, water, or air to survive.  Relatedly, do they age or sleep?

They disappear into nothingness when killed, but reform bodies when summoned again. Spirits are not defined anywhere that I'm aware of, but they almost sounds like elemental beings.

EDIT: Difficult to say. Some celestial and fiends need food and air. Others do not.

I like your comparison with elemental, though.

Because familiars are created by a spell, and spells do only what they explicitly say they do... the answer comes down to whether or not the requirement to breathe and eat/drink is considered to be part of "the statistics of the chosen form."

Which is a real fancy way of me saying the rules aren't entirely explicit, so each group should decide for themselves. In my group, familiars don't need to be fed (paying 10 gp each time you need to cast find familiar is enough, I don't feel a need to add to the book-keeping and costs that are already happening to keep the PCs fed), but they do have to breathe (so they aren't granted an unmentioned immunity to suffocation or inhalation hazards like poison gas) - but familiars can eat, and it makes them very happy to be fed special treats by the person they exist to assist.

Id say theyd need air as some have water breathing otherwise im not sure bout nutrition

The familiar is given the statblock of the creature form it is taking.

If the creature statblock indicates it does not need nutrition or air then it doesn't. Otherwise, it follows the rules of such as any other creature does, since familiars are creatures while they have a form.

I would say that when summoned, the familiar's needs are those of a normal animal of the  chosen form. Essentially, the spirit you have summoned is embodied in that form and can be treated as a normal animal while it is with you.

However, if and when you choose to "temporarily dismiss your familiar into a pocket dimension", then I would treat it as effectively being in stasis while in said dimension.  Thus, while in the pocket dimension it would not need food, water, air etc.  nor would it age.

In a similar vein, I've made rulings (although a bit unpopular at the table) where when a player left their familiar out in the rain all night so that they wouldn't lose track of their quarry's footprints, that their familiar got sick (and I marked it down as an instance of mistreatment for the familiar). They tried to argue that since it was a spirit, that it couldn't get sick, which then led to an in depth discussion about how the familiar would be allowed to interact with the physical world.

In short, I allow PCs to include feeding the familiar as part of their implied or assumed actions in a day, and the PC just shares a bit of their daily ration. Doesn't really add to any expenses that way, and still encourages PCs to treat their familiars with respect.

Quote from xeonix22 >>

In a similar vein, I've made rulings (although a bit unpopular at the table) where when a player left their familiar out in the rain all night so that they wouldn't lose track of their quarry's footprints, that their familiar got sick (and I marked it down as an instance of mistreatment for the familiar). They tried to argue that since it was a spirit, that it couldn't get sick, which then led to an in depth discussion about how the familiar would be allowed to interact with the physical world.

In short, I allow PCs to include feeding the familiar as part of their implied or assumed actions in a day, and the PC just shares a bit of their daily ration. Doesn't really add to any expenses that way, and still encourages PCs to treat their familiars with respect.

This would an example of homebrewing. The spell doesn't really outline that any of this should happen, so you're essentially coming up with your own house rules. There isn't anything wrong with that, but I generally advise DMs that if they're going to homebrew additional rules that they should probably inform their players of these changes in advance before it negatively impacts them. Otherwise you may end up leaving them feel treated unfairly, because they thought the rules worked the way they say they do in the books and you surprise them with consequences for not having read your mind about changes you'd like to implement.

Quote from Ravnodaus >>

This would an example of homebrewing. The spell doesn't really outline that any of this should happen, so you're essentially coming up with your own house rules. There isn't anything wrong with that, but I generally advise DMs that if they're going to homebrew additional rules that they should probably inform their players of these changes in advance before it negatively impacts them. Otherwise you may end up leaving them feel treated unfairly, because they thought the rules worked the way they say they do in the books and you surprise them with consequences for not having read your mind about changes you'd like to implement.

In fairness, this entire thread so far is in the realm of homebrew rulings. The spell doesn't deal with food or air consumption either, but it's one of those things that should be addressed for players that plan on using familiars in ways outside of the norms.

Typically I agree with you about informing players prior to these kinds of rules, but for some background information the player involved in this ruling was notorious for coming up with his own homebrew character features (and putting them on his character sheet without asking for approval), as well as misinterpreting RAW to try to be in his favor. As a result, it left the other players at the table feeling slighted, but our group is decently close as friends so no one wanted to "hurt his feelings." Since he and I take turns DM'ing (part of where his problem stems from), we weren't keen on the idea of kicking him out of the group, and I didn't think it was fair to just narratively meteor his character to death.

Basically the solution so far has been to issue in-game "warnings" when he starts to go too far, to give him a chance to rethink before fully committing. In the sick familiar example, after about an hour of it sitting outside, I told the player that his familiar had started to sniffle, and eventually sneezed. He left it out there anyway, and I told him that it was now sick, but there were no actual consequence to it at that point, other than a realization that he probably shouldn't do that again. That was where his argument started, but I told him that it'd only be sick for a day, and if he continued not to care for it that it would either take penalties (the actual homebrew ruling there), or that it would start to consider breaking its contract with him (actual RAW ruling).

I get your point though, it's not fair to come up with arbitrary rulings at the table without any kind of forewarning.

Quote from xeonix22>>

Basically the solution so far has been to issue in-game "warnings" when he starts to go too far, to give him a chance to rethink before fully committing. In the sick familiar example, after about an hour of it sitting outside, I told the player that his familiar had started to sniffle, and eventually sneezed. He left it out there anyway, and I told him that it was now sick, but there were no actual consequence to it at that point, other than a realization that he probably shouldn't do that again.

I'm getting a strong vibe that you just want to control his character and are creating restrictions that don't exist to soft-force him into compliance. The bolded bit is what reveals this. You are trying to teach him how you want him to behave, and this isn't going to go well, for either of you.

Maybe you're not. But inventing an illness that affects only familiars who are exposed to rain is... unusual.

Diseases are caused by being exposed to a contaminant, and rain isn't typically considered such a thing. Unless you have the most disease ridden homebrewed world imaginable. Or that was some sort of homebrewed necromantic disease storm, then causing a disease from inclement weather is extremely problematic.

That was where his argument started, but I told him that it'd only be sick for a day, and if he continued not to care for it that it would either take penalties (the actual homebrew ruling there), 

The familiar is a creature. If you want it to suffer penalties from being exposed to the weather... just... use the rules for harsh weather. You don't need to reinvent the wheel here. And you certainly don't need to apply a disease from literally out of thin air.

If the weather was actually extremely cold, which TBH it doesn't sound like it was, but if it was:

Extreme Cold

Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain one level of exhaustion. Creatures with resistance or immunity to cold damage automatically succeed on the saving throw, as do creatures wearing cold weather gear (thick coats, gloves, and the like) and creatures naturally adapted to cold climates.

This is the rules for extreme cold. Applying this ruling to a familiar is perfectly RAW. Though, some forms familiars take could indeed qualify as "adapted to cold climates".

or that it would start to consider breaking its contract with him (actual RAW ruling).

This isn't RAW at all. The familiar doesn't get to break the contract. There isn't even a contract from which they could break. It is simply a loyal servant that obeys all commands.

I'm getting a strong vibe that you just want to control his character and are creating restrictions that don't exist to soft-force him into compliance. The bolded bit is what reveals this. You are trying to teach him how you want him to behave, and this isn't going to go well, for either of you.

Maybe you're not. But inventing an illness that affects only familiars who are exposed to rain is... unusual.

or that it would start to consider breaking its contract with him (actual RAW ruling).

This isn't RAW at all. The familiar doesn't get to break the contract. There isn't even a contract from which they could break. It is simply a loyal servant that obeys all commands.

I'd be happy to discuss it with you further off forum posts, since it's getting a bit off track here. :)

But as far as the contract part, character has an imp familiar, which does have this trait, making it RAW:

Familiar. The imp can enter into a contract to serve another creature as a familiar, forming a telepathic bond with its willing master. While the two are bonded, the master can sense what the imp senses as long as they are within 1 mile of each other. While the imp is within 10 feet of its master, the master shares the imp’s Magic Resistance trait. If its master violates the terms of the contract, the imp can end its service as a familiar, ending the telepathic bond.

Imp, Quasit, and Pseudodragon familiar variants (along with a handful of others) happen to have this trait. Player is aware of that, so it's not like he's being tricked.

Quote from xeonix22 >>

But as far as the contract part, character has an imp familiar, which does have this trait, making it RAW:

Familiar. The imp can enter into a contract to serve another creature as a familiar, forming a telepathic bond with its willing master. While the two are bonded, the master can sense what the imp senses as long as they are within 1 mile of each other. While the imp is within 10 feet of its master, the master shares the imp’s Magic Resistance trait. If its master violates the terms of the contract, the imp can end its service as a familiar, ending the telepathic bond.

Imp, Quasit, and Pseudodragon familiar variants (along with a handful of others) happen to have this trait. Player is aware of that, so it's not like he's being tricked.

 It seem you are confusing two different things here. A contract Imp (familiar variant) familiar is a different thing from a familiar gained from the Find Familiar spell. And a contracted Imp (familiar variant) doesn't use any of the extra rules of a Find Familiar familiar and vice versa (and this holds true even if a Warlock uses a pact feature to get a Imp familiar).

A Find Familiar familiar, which the thread was talking about and you seemed to be using too, has no contract to break so that cannot ever happen. I can see potential for RP here but that is, as they say, a completely different discussion.

Quote from Thezzaruz >>

 It seem you are confusing two different things here. A contract Imp (familiar variant) familiar is a different thing from a familiar gained from the Find Familiar spell. And a contracted Imp (familiar variant) doesn't use any of the extra rules of a Find Familiar familiar and vice versa (and this holds true even if a Warlock uses a pact feature to get a Imp familiar).

A Find Familiar familiar, which the thread was talking about and you seemed to be using too, has no contract to break so that cannot ever happen. I can see potential for RP here but that is, as they say, a completely different discussion.

That's actually really interesting, I didn't know that bit. I have a feeling the player doesn't either, since he's the one that brought the familiar variant to the table. It also drastically changes the rules for how he should be allowed to play using his familiar, because up until now he has been using the familiar as long distance telepathic communication for the party (way outside of 100 ft away), and to gain its magic resistance trait, both of those being in the variant version. His whole class shtick revolves around this particular familiar, which is part of the reason I made the decision to narrate that it got sick in the first place.

If this familiar that you use is the core behind your character, and all of your RP is behind how "special" it is and how much it means to you, I figure you should at least try to treat it right. But if the familiar variant isn't even the one you're supposed to use for the Pact of the Chain (or in his case, a customized pact that does the same thing plus extra), then it definitely saps how much extra power he gets from using it.

Just to be clear. Any PC could have a contracted Imp (familiar variant) familiar as long as the DM approves it. I would assume that it would require some sort of questing to find and befriend the Imp before a contract becomes an option.

And any PC that gets his hand on the Find Familiar spell can get a "regular" familiar. And if that PC is a Chainlock (or have a DM approved custom pact as in your case) he could then have that familiar take the form of an Imp.

And a PC could conceivably do both and end up with two Imp familiars but with different rules for how they work. I wouldn't recommend it but it should be possible.

Quote from xeonix22 >>

That's actually really interesting, I didn't know that bit. I have a feeling the player doesn't either, since he's the one that brought the familiar variant to the table. It also drastically changes the rules for how he should be allowed to play using his familiar, because up until now he has been using the familiar as long distance telepathic communication for the party (way outside of 100 ft away), and to gain its magic resistance trait, both of those being in the variant version. His whole class shtick revolves around this particular familiar, which is part of the reason I made the decision to narrate that it got sick in the first place.

 If it is that integral to the character then I would have a discussion with the player to decide how to continue forward, changes might have to be made that will have a profound effect on how the character works and that could be problematic to the concept as a whole.

But yea, if the familiar is integral to the core of the character then I'd expect the roleplay to match that.

Quote from Thezzaruz >>

Just to be clear. Any PC could have a contracted Imp (familiar variant) familiar as long as the DM approves it. I would assume that it would require some sort of questing to find and befriend the Imp before a contract becomes an option.

And any PC that gets his hand on the Find Familiar spell can get a "regular" familiar. And if that PC is a Chainlock (or have a DM approved custom pact as in your case) he could then have that familiar take the form of an Imp.

And a PC could conceivably do both and end up with two Imp familiars but with different rules for how they work. I wouldn't recommend it but it should be possible.

  If it is that integral to the character then I would have a discussion with the player to decide how to continue forward, changes might have to be made that will have a profound effect on how the character works and that could be problematic to the concept as a whole.

But yea, if the familiar is integral to the core of the character then I'd expect the roleplay to match that.

I'll definitely be having a chat with him, and probably give him the option to keep the variant familiar (social contract and all), but to ensure that he knows it only gets one life and can't be pocket dimensioned/resummoned. Mostly because the familiar was written into his backstory as a creature that he befriended, and he just mechanically wanted it as a familiar. Since he's a pseudo-Chainlock, with matching invocations, I don't really think it's fair to strip him of all of the benefits offered, but like you said, he could just summon a "normal" familiar too. It also puts some extra RP value on their relationship, where he wouldn't just send it blindly into combat without careful consideration.

It's also important to note that I only approved his homebrew pact on the condition that if it became problematic at the table, I reserved the right to make adjustments as needed if it was making the game not fun for the other players.

Yeah, the thing is the Imp (familiar variant) is actually an Imp - a devil able to make such contracts.

A familiar from Find Familiar, even when a chainlock familiar with an Imp form, is not a real Imp. It's a spirit you have bound into an Imp form. 

Not to pile on here, but if it's an actual Imp, as opposed to a familiar, it seems odd that a devil can get sick from being out in the rain. I mean, it seems odd that any kind of spirit can get sick, but an actual devil, who has resistance to cold damage, it doesn't seem they'd be too bothered by getting wet. I could definitely see it being annoyed, and angry, and then maybe faking sick as a passive-aggressive way to express its displeasure, though.

Quote from Xalthu >>

Not to pile on here, but if it's an actual Imp, as opposed to a familiar, it seems odd that a devil can get sick from being out in the rain. I mean, it seems odd that any kind of spirit can get sick, but an actual devil, who has resistance to cold damage, it doesn't seem they'd be too bothered by getting wet. I could definitely see it being annoyed, and angry, and then maybe faking sick as a passive-aggressive way to express its displeasure, though.

 Technically it's RP reskinned as a flying dog familiar, it's not impy in any way apart from its statblock. He just wanted that because of invisibility, telepathy, and magic resistance. You could argue that dogs are also meant to be outside, but in a situation where the owner says "go sit outside in this uncovered area for 8 hours in this raging thunderstorm while we hang out in this house", I was less inclined to make it a favorable ruling.