The description of the bag of holding in the DMG (p. 154) states:
Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures [...]
Is there any RAW way to create sustainable conditions for breathing creatures inside a bag of holding (or other similar extradimensional spaces, for that matter)?
...IF you're asking if there's a RAW way for someone to survive inside a Bag of Holding for longer than 10 minutes. If you're asking if it's possible to make a livable environment inside the Bag, then the answer is 'Not by RAW'.
Thanks to the newly released Spelljammer optional supplement, there are now two ways for breathing creatures to survive within a Bag of Holding.
The first method is by wearing a Necklace of Adaptation. Its description says:
While wearing this necklace, you can breathe normally in any environment...
The text does say 'any environment', so a RAW interpretation would mean literally any environment...and that would include the insides of a Bag of Holding!
The second method is by use of the Air Bubble spell found in the Spelljammer supplement:
You create a spectral globe around the head of a willing creature you can see within range. The globe is filled with fresh air that lasts until the spell ends.
Of course, any DM could still rule otherwise...but its clear to me that by the RAW interpretations above that these are both viable means of surviving inside a Bag of Holding.
Actually the interior is plenty big enough for a room inside, holding up to 64 cubic feet, or an 8×8 cube room, the item description reads that the outside dimensions are 2 feet at the mouth and 4 feet deep, while the inside is much bigger. Head Canon here but I always pictured this as a kind of spandex bag on the inside, stretching to accommodate the things inside, up to a point, at which it tears. There are several options to stop yourself from suffocating in the bag, but here are a few of my favorite. The warforged is the easiest option, being nonliving and non breathing race you can just dive in and be fine, however if you want to use a different race there is still water breathing, but you couldn't just fill the bag up. It won't hold that much liquid. So what you can do is use a glass blowing kit or tinker's tools to make a fish bowl helmet, or commission someone else to do it. Then get hold of one of the magic items that give you water breathing (my favorite is the one that puts a bubble of air around your head while you're submerged) Then build an 8×8 a frame inside the bag using wood. Keep in mind the floor and walls are made of cloth and can be punctured, also, including yourself, the bag can only hold 500 lbs, but I think a bag of holding house sounds awesome and a good dm should allow you to alter several bags you've created, to be connected and sewn together to create a house with several rooms. Also remember that each bag is 2×4 feet and weighs 15 lbs, so if you have more than 3 or 4 most characters would have a difficult time carrying them. But it could be in a wagon, possibly giving it a more sturdy floor, or hanging from a horse or other mount.
It is not clear to me why you would not suffocate after 10 minutes divided by the number of creatures, no matter what, because the bag states:
Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate.
There is no condition on the suffocation. It does not matter if you have sources of fresh air or whatever, the bag does not say you suffocate because you used up the air.
We read that into the bag, but that is not what it says. It also would take a lot longer than 10 minutes before you use up all the are in a 64 cubeic feet space and suffocate for lack of air. A typical human needs about 450 liters of air in an hour. 64 cubic feet are over 1,800 liters. You would survive about four hours in such a space before you run out of air. So it cannot be that you suffocate because you run out of air after just 10 minutes.
If that is not it, all that remains that you suffocate after 10 minutes if you are a breathing creature because ... magic. The bag will suffocate you. Finding other ways to breathe does not really change it. You are still a breathing creature, and your 10 minutes still run out, no matter if you have a bubble of air, or if you have a neclace of adaption. You are in the bag, and you will begin to suffocate.
PS. If we assume magical ways to breathe work to get around the bag's restricitons: Fill the bag partially with water, depending on your own weight, maybe 50 gallons or so, to keep under the weight limit, which will fill about half a foot at the bottom. Cast water breathing on yourself each day. Enjoy your new, wet home.
Water breathing is a ritual that does not even cost you a spell slot, runs for 24 hours and states:
This spell grants up to ten willing creatures you can see within range the ability to breathe underwater until the spell ends.
You will be able to breathe underwater for the next 24 hours, no air from the bag needed. It is however another question how habitable this is. You get kind of soggy with time.
There is also the option of a pair of ventilating lungs (E:RftLW, p. 279) in the Eberron campaign world. They replace your normal lungs and allow you to breathe in a complete vacuum, as well as granting protection from several gas spells and the ability to create a gust of wind effect once per short rest.
Or...
Have a warforged character. Nothing like having a warforged in a bag. Surprise!
The description of the warforged's Constructed Resilience trait states, in part (E:RftLW, p. 36; WGtE, p. 70):
- You don’t need to eat, drink, or breathe.
