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Casting Demiplane allows the caster to have:

However, my problem is that the created demiplane is too small and barren. I want more space for activities. It might be possible with mordenkainen's magnificent mansion, but I'm not sure if it even works, and the duration is limited (24 hours) without some way to affect the passage of time in the demiplane.

So, is there any way to add or expand —ideally permanently— to the space-size of this 30' × 30' demiplane?

Answers may include: any of the sourcebooks (PHB, Volo's, Mordenkainen's, etc.), tweets from Lord Crawford, Sage Advice, any 5e Unearthed Arcana... or whatever one can find that is RAW, quasi-legal or even slightly more tested than outright home-brew.

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    \$\begingroup\$ I trimmed out a lot of commentary, etc. I don't think are needed at all, but wanted specifically ask you about the line "The 5e spell specifies the new parts must be cast OUTSIDE of the original" because I have no idea what it is trying to say. Reminder that you can see the revisions if you need to see the whole of what you wrote. \$\endgroup\$
    – Someone_Evil
    Commented Jun 10, 2020 at 0:55
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    \$\begingroup\$ @Someone_Evil : my thanks. For someone who claims to be evil, you are most considerate and rather kind. Don't worry, your secret is safe with me. \$\endgroup\$ Commented Jun 10, 2020 at 3:11
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    \$\begingroup\$ Are you a Dungeon Master or a player? Depending on that your answer may be more based on the interpretation of rules or obscure other rules or spells in the sources you outlined. Also, what are you using the Demiplane for that needs extra space, and how much extra space does it need? \$\endgroup\$
    – Bazza491
    Commented Jun 12, 2020 at 2:12
  • \$\begingroup\$ @Bazza491 I wanted, as a DM, to have Fraz-Urb'luu to have a logical and RAW ability to make planes over time. It looks like i will be twisting the rules a bit. Thanks for asking though! \$\endgroup\$ Commented Jul 3, 2020 at 1:39

2 Answers 2

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Demiplane is a very limiting spell, and only allows you to create a 30'x30'x30' space or connect to another known space created by another casting of demiplane. You can't even connect to demiplanes created in other ways.

There does not appear to be any way to add or expand to this space, although the spell does say that the walls are actually made of material, which means that at GM discretion it may be possible to excavate a larger area, either by hand or with disintegrate. Depending on what exactly you are looking for this may be your only option.

Demiplane does allow you to create more than one such 30'x30'x30' space, however considering it is an 8th level spell and you can only connect to 1 demiplane at a time for 1 hour at a time, this may or may not be useful.

As you mentioned, mordenkainen's magnificent mansion creates a much larger space, but this space is temporary, and ceases to exist entirely while the spell is not in effect. Considering the spell has a duration of 24 hours, it is feasible to cast the spell continuously indefinitely. It is also possible to use a spell such as plane shift to gain access to a demiplane created in some other way, such as by a deity.

Finally there is the Rod of Security, which is a very rare magic item that allows you to create an extraplanar space of your own description that lasts for 200 days divided by the number of creatures present, and can be recast after 10 days. This effect has no restriction on the size of the extraplanar space, and has the added benefit of suspending aging while you are in the space.

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    \$\begingroup\$ "the spell does say that the walls are actually made of material, which means that at GM discretion it may be possible to excavate a larger area": I don't know, if I made a hole in the wall, I'd be worried that I would find water pouring through. Or that I would see eyes. \$\endgroup\$ Commented May 20, 2022 at 21:00
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    \$\begingroup\$ I suppose the DM could place a Rod of Plane Shift, in each Demiplane; defining each Rod as targeting a specific Teleportation Circle in another Demiplane (or use Living Plane Shift Spells, to similar effect). That seems viable, RAW? Making the Rod immune to planar travel, would be homebrew, but I believe casting Plane Shift via reusable magic items, would be one canonical way to establish connections between Demiplanes? It still won't enlarge each 'room', & use of a high rarity magic item at every "exit" (& a permanent Teleportation Circle for each one to target) is extremely costly, but... 🤷 \$\endgroup\$ Commented Jul 19, 2022 at 20:04
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In reference to expanding this Room's Dimensions; you could theoretically make a series of 30×30×30 Demiplanes lined up next to each other in any direction, then just rip their barriers apart to connect them; a powerful enough caster could likely perform this without too much effort. Phlogiston is what technically makes the creature/object ethereal and provides omnidirectional movement (2 separate effects), if you are paying attention you will recognize the reaction to “willpower” (Intelligence, Mental Fortitude, Mental Prowess). Strong Intelligence when visiting the Deep Ethereal using Demiplane Spell will give you a stronger/faster character amidst the Phlogiston.

  • Possible to connect these rooms? Yes.
  • Easy to connect these rooms? Nope.

Supporting and related references to source material can be found below.


I am going to pull from my gathered information listed in this Stack Exchange question over here to answer your original question very quickly yet extremely thoroughly, though first I will preface a few things:

  • I will preface stating I presume you are using the Great Wheel Cosmology
  • Mordenkainen's Magnificent Mansion doesn't expand space, this would actually get Conjured, unlike the Demiplane, you wouldn't die from this, just a regular portal to a temporary pocket dimension.
  • in the Deep Ethereal (movement, breathability, travel)
  • from “Phlogiston” (D&D equivalent of “Space Ether”, AKA Protomatter)
  • No, you can't merge the Doors (you would require multiple mages to cast 4× anyway)
  • You forgot Squeezing, you should actually update your question to reference “No Elephants” as opposed to “No Ogres” because Ogres (& donkeys) could squeeze their way past this door.

Now, onto my main bulk reference citation to help thoroughly answer your original question (I looked at this question's Edit History), again; presuming you are using the Great Wheel Cosmology:


Page 48 in the DMG reads:

Ethereal Plane:

...

The Ethereal Plane is a misty, fog-bound dimension. Its "shores," called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. Visibility in the Border Ethereal is limited to 60 feet. The plane's depths comprise a region of swirling mist and fog called the Deep Ethereal, where visibility is limited to 30 feet.

Characters can use the etherealness spell to enter the Border Ethereal. The plane shift spell allows transport to the Border Ethereal or the Deep Ethereal, but unless the intended destination is a specific location or a teleportation circle, the point of arrival could be anywhere on the plane.

Border Ethereal:

From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane. Conversely, the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic.

Normally, creatures in the Border Ethereal can't attack creatures on the overlapped plane, and vice versa. A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal. The exceptions are certain magical effects (including anything made of magical force) and living beings. This makes the Ethereal Plane ideal for reconnaissance, spying on opponents, and moving around without being detected. The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.

Deep Ethereal:

To reach the Deep Ethereal, one needs a plane shift spell or arrive by means of a gate spell or magical portal.

Visitors to the Deep Ethereal are engulfed by roiling mist. Scattered throughout the plane are curtains of vaporous color, and passing through a curtain leads a traveler to a region of the Border Ethereal connected to a specific Inner Plane, the Material Plane, the Feywild, or the Shadowfell. The color of the curtain indicates the plane whose Border Ethereal the curtain conceals; see the Ethereal Curtains table.

...

Traveling through the Deep Ethereal to journey from one plane to another is unlike physical travel. Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it's impossible to measure speed and hard to track the passage of time. A trip between planes through the Deep Ethereal takes 1d10 × 10 hours, regardless of the origin and destination. In combat, however, creatures are considered to move at their normal speeds.


From this Data we found:

The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.

Additionally, this extra information prevents certain results: Exertion to attempt to destroy Walls gets limited from:

Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it's impossible to measure speed and hard to track the passage of time.


Depending on if you follow specifically Forgotten Realms Lore, you would find this infodump on the Ethereal Plane useful:

Cosmology: Great Wheel

Great Wheel Cosmology Map

In the Great Wheel cosmology model, the Ethereal plane existed adjacent to the Prime Material Plane and connected it to the Inner planes (the Elemental planes plus the Energy planes). The Ethereal touched the Prime at all points that were located within crystal spheres through what was called the Border Ethereal. It was unknown whether the Border Ethereal touched the Prime in the phlogiston Citation: 6 Citation: 13.

The non-Border region was called the Deep Ethereal. While in the Border Ethereal, a traveler could still see into the adjacent plane but only dimly and not very far, whereas those on the bordering plane could not see the traveler without magical detection. Verbal communication was not possible between the Border Ethereal and the bordered plane. The Ethereal was unique among the many planes in that an individual could exist in two planes simultaneously: the Border Ethereal and the adjacent plane Citation: 6.

When travelers crossed into the Border Ethereal, they and all their possessions were converted to their Ethereal equivalents—metal became ethereal metal, flesh became ethereal flesh, and so on Citation: 14 —allowing free movement (in most cases) in any direction through the solid matter of the adjacent plane Citation: 6. Since everything was permeated with ethereality, an air-breathing creature could breathe ethereal air and could not drown in an ethereal lake nor be crushed by an ethereal rock. However, not all in the adjacent plane was insubstantial. Living things larger than one-celled animals generated an aura that radiated around them and prevented passage to their interior, so an ethereal traveler could not place a weapon inside a living creature where it would materialize and cause damage. A jungle would be an extremely torturous maze to navigate; it would be much easier to float above the vegetation or pass below its roots. Dense metals such as lead or gold also prevented passage of ethereal matter. And finally, some magic spells and alchemical mixtures could form an effective barrier Citation: 14.

Travel in the Ethereal and Border Ethereal was accomplished by force of will—you wished to go somewhere and you did, at your normal rate of movement. There was a sense of up and down but no real gravity existed. Objects released from possession would hover where they were dropped; it was impossible to fall in the Ethereal plane Citation: 15. To get to another plane, one had to pass through a curtain of vaporous color into the Deep Ethereal, then traverse that region until reaching the curtain that demarcated the Border Ethereal of the destination plane. Each Inner plane and demiplane had a curtain with a unique color. The Prime Material plane's curtain was turquoise Citation: 14. If the (usually magical) ethereal effect wore off while a traveler was in the Deep Ethereal, he would immediately be forced through a random curtain at a random location in the Border Ethereal and deposited on the plane which it bordered Citation: 15.

When you passed through a curtain into the Deep Ethereal, time slowed down to one tenth the rate it flowed in the Border Ethereal and the plane that it bordered. For every ten hours spent in the Deep Ethereal only one hour passed on the other side of the curtain. Metabolic and other natural processes slowed down also, so it did not feel like ten hours, when you crossed back through a curtain into a Border Ethereal you were only an hour older and an hour hungrier Citation: 14.

The Deep Ethereal swirled with large blobs of protomatter (imagine a cosmic lava lamp) which could form a demiplane when a critical size was reached. Powerful wizards, technologists, or demigods could also bend the proto-matter to their will and create a demiplane Citation: 16. These nascent planes might exhibit some of the characteristics of the Inner planes or the Prime, but with their own rules of gravity, material make-up, etc., and even support life. Most demiplanes eventually collapse into themselves and break up or merge with another Inner or Prime Material plane Citation: 17.


Citation: 24 in case you want to Review my source referencing these citations.

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    \$\begingroup\$ I'm not the downvote, but I'm having a hard time following this answer. You should probably trim the quotes down to only the parts relevant to your conclusion. I'm not sure where the leap from ethereal plane to connecting demiplanes are, nor even how the ethereal plane is relevant. Additionally, please don't signal your edits (there's the revision history you're already aware of for that) and you may have interest in reading: Why is an answer being downvoted without any comments? \$\endgroup\$
    – Someone_Evil
    Commented May 20, 2022 at 19:52
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    \$\begingroup\$ @BlueShocker64 I did downvote this as I see no clear evidence supporting (1) that you can connect multiple demiplanes, (2) that demiplanes are in the ethereal plane, (3) that phlogiston is a thing in 5e, (4) how citations from 1987 (long before 5e) are relevant to all of this. This is not to say all of them may be the case, but I am unable to tell from your answer. As Someone_Evil suggested, it might help if you limit the quotations to the relevant parts, if you are able to support these claims, I am happy to revert this to an upvote. Thank you for contributing. \$\endgroup\$ Commented May 20, 2022 at 20:50
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    \$\begingroup\$ It isn't clear how the conclusions of this answer are derived from the premises. Also the demiplanes description in the DMG has demiplanes in the Other planes section. That section states that these planse are "A variety of realms exist between or beyond the other planes", which contradicts the assumption that a demiplane is part of the ethereal plane. \$\endgroup\$
    – GcL
    Commented May 20, 2022 at 21:00
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    \$\begingroup\$ @BlueShocker64 I have found the answer in my materials. That's why I used it to point out your error. If you want relevant citations, the fact that phlogiston is a feature of the prime material plane can be found on pages ten and eighteen of Spelljammer - The Concordance of Arcane Space. I have not been able to find any citation that makes any reference to phlogiston being a feature of the astral or ethereal planes - unless you're aware of one that I'm not? \$\endgroup\$
    – GMJoe
    Commented Jul 24, 2022 at 23:58
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    \$\begingroup\$ @BlueShocker64 Do you have a specific reference I could look at? \$\endgroup\$
    – GMJoe
    Commented Aug 23, 2022 at 0:23

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