Casting Demiplane allows the caster to have:
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A Medium-only door ('Must be this size or smaller to ride – NO OGRES ALLOWED') that vanishes when spell ends / after one hour.
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Creates a permanent (30' × 30') room of stone &/or wood walls.
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I know that customizations such as gravity are dependant on DM discretion.
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And that it can be reconnected to if known or using a tuning fork.
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.
Best Answer
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.