I was wondering if I could use demiplane as a trap. Can I end the spell whenever I want? It's not a concentration spell and it lasts an hour. Could I get a dragon to enter the 30x30 room, slam the door and end the spell thus trapping the dragon in the demiplane for eternity? Or does the spell last for an hour no matter what?

The rules aren't clear on whether you can end a non-concentration spell voluntarily.  I'd default to saying that if the spell doesn't have a clause for letting you end it before the duration runs out (which is the case for Demiplane), you'd need to use Dispell Magic or a similar effect.  And even without that, tricking a dragon into entering the Demiplane would be difficult since the door the spell creates is only large enough for medium sized creatures to fit through it, any dragon larger than that is unlikely to voluntarily try to go through such a portal.  And if you're slinging 8th level spells around, you don't need to be concerned about any dragons of medium size or smaller.

FYI: It’s been clarified in Sage Advice that if a spell is not concentration, you cannot end it unless it specifically says you can or if you use dispel magic.

Can a spellcaster dismiss a spell after casting it?

You can’t normally dismiss a spell that you cast unless (a) its description says you can or (b) it requires concentration and you decide to end your concentration on it. Otherwise, a spell’s magic is unleashed on the environment, and if you want to end it, you need to cast dispel magic on it.

So demiplane lasts the full duration unless someone dispels it. So you would need to 1) wait 59 minutes and 54 seconds, and 2) get the dragon to fit in there as noted above.

Thanks. Makes demiplane not quite as useful. I was thinking of it as a trap or at least a very large bag of holding. But I guess not.

Demiplane is a super-fantastic trap spell. But it requires making it in advance and the use of Glyph of Warding. Maybe store a lot of Flesh to Stone spells to go off every so often on the target so the chances of them succumbing are pretty high - turning them to stone forever.

If you know the target cannot use Planar Travel then you can make a Demiplane, go in, wait for the door to close. Cast Gate to Summon the target (there's no save) and immediately end concentration as soon as it appears. Then cast Plane Shift on yourself to escape and leave the target trapped forever. 

Quote from Cyb3rM1nd >>

Demiplane is a super-fantastic trap spell. But it requires making it in advance and the use of Glyph of Warding. Maybe store a lot of Flesh to Stone spells to go off every so often on the target so the chances of them succumbing are pretty high - turning them to stone forever.

 

If you know the target cannot use Planar Travel then you can make a Demiplane, go in, wait for the door to close. Cast Gate to Summon the target (there's no save) and immediately end concentration as soon as it appears. Then cast Plane Shift on yourself to escape and leave the target trapped forever. 

 

 

 

 

 I definitely agree with this strategy. Shove an enemy into the room, and let all those fancy Glyphs and spells go off in their face, as you shut the door on them. Classic.
Also, keep in mind that, when the spell ends, it's only the door that disappears. That same Demiplane can be re-accessed much later, via a future casting.

I feel like using it as a trap isn't the major play though. Using it as a Clone Repository, however, is a much more viable move, especially if you grabbed Plane Shift. For example:
Step 1: Cast Demiplane.
Step 2: Place Clone inside.
Step 3: ???
Step 4: Respawn in Demiplane.
Step 5: Plane Shift to a safe location, back on whatever plane you have access to.

To prevent people from scrying your Demiplanes, in order to destroy your Clones and/or steal your backup loot, I'd recommend further steps:
Bonus Step 1: Purchase a bottle of invisible ink, and a blindfold.
Bonus Step 2: Step into Demiplane and blindfold yourself, not necessarily in that order.
Bonus Step 3: Splatter the invisible ink around all willy-nilly, paying absolutely no attention to anything whatsoever.
Bonus Step 4: Plane Shift back to town, and remove the blindfold.
Bonus Step 5: Throw away the invisible ink, making sure NOT to pay attention to how much is left.

While these bonus steps sound silly, they're very important, due to this section of Demiplane's spell description: "Additionally, if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead." Since even you don't know the nature of the pattern of ink splatters made, or how much ink was used while blindfolded, there's no easy way to search your memories for the exact specifics required. You, however, have full access to that Demiplane, because you're the one who made it. The Divination Wizard who decided to scry your mind, saw a room that housed only a single Clone, and thought he could jump into your Demiplane with that info alone? Well... He'll probably land near your Demiplane. Considering where Demiplanes are usually located, however... well... "Close" isn't necessarily close enough.