How to approximate with RAW...

[Avigor]

A magic item, preferably usable at will: a miniature door you can throw or place against a wall, which then expands into a proper door opening into a large, multi-story extra-dimensional space (I'm thinking 30' cube but that's flexible), where the structure is set but the contents must be put into it by the user and are consistent from visit to visit.

In other words, a Leomund's secret chest spell on steroids and in item form, tied to a door instead of a chest.

[Deadline]

A magic item, preferably usable at will: a miniature door you can throw or place against a wall, which then expands into a proper door opening into a large, multi-story extra-dimensional space (I'm thinking 30' cube but that's flexible), where the structure is set but the contents must be put into it by the user and are consistent from visit to visit.

In other words, a Leomund's secret chest spell on steroids and in item form, tied to a door instead of a chest.

So ... Mordenkainen's Magnificent Mansion, but in item form?

[Bronk]

This sounds a lot like the Enveloping Pit from MIC, although the dimensions are 10 feet wide by 50 feet deep.

Edit: I think the trick would be to have multiple enveloping pits inside the first one, so that you could end up having a 5 story structure inside, with each story having up to two 100' hallways or towers/basements, depending on orientation.

[Avigor]

So ... Mordenkainen's Magnificent Mansion, but in item form?

Only if the mansion doesn't puke everything out or wipe it from existence or cast the contents adrift in the astral/ ethereal plane or whatever whenever the door is closed and collapsed into the tiny door figur and the item is basically just a mobile portal to a permanent mansion effect. Also I wasn't exactly intending it to include free contents, whether food, unseen servants, or whatever, so that's actually overshooting the goal in a way to create a custom slotless item of permanent Mordenkainen's magnificent mansion, even if that could work.

This sounds a lot like the Enveloping Pit from MIC, although the dimensions are 10 feet wide by 50 feet deep.

Edit: I think the trick would be to have multiple enveloping pits inside the first one, so that you could end up having a 5 story structure inside, with each story having up to two 100' hallways or towers/basements, depending on orientation.

First problem, that specifies that it opens on floors (unlike the Portable Hole), suggesting it won't work on a wall. Second problem, it is a relic tied to a deity, which could cause complications. Third problem, given the RAW rule of what happens when a bag of holding and a portable hole interact, there are possible issues with using multiple of either Enveloping Pit or Portable Hole to create a multi-story extra-dimensional structure, depending on how the DM rules it.


I'm now kinda thinking it might be best to just negotiate with a DM for a homebrew item that lists both Mordenkainen's magnificent mansion and Leomund's secret chest as prerequisite spells... But I would still like to know if there are any actual RAW means of pulling this off.

[Crake]

I'm now kinda thinking it might be best to just negotiate with a DM for a homebrew item that lists both Mordenkainen's magnificent mansion and Leomund's secret chest as prerequisite spells... But I would still like to know if there are any actual RAW means of pulling this off.

Nothing about custom magic items is RAW, it's all guidelines for the DM to look at and assess if a) the item is something he wants in his setting, and b) how much the item should cost based on what it can do.

You could instead base the spell on rope trick rather than magnificent mansion, if you don't want all the extras that come wiht magnificent mansion, but rope trick does have a maximum capacity of 8, so if that's an issue for you, it might not be feasable. Rope trick doesn't actually specify a volume, but it can contain 8 creatures of any size, so 8 colossal creatures could get up in there, so it's at least that big. Refluffing the rope as like, a door handle or something, and having the entrance be vertical rather than horizontal shouldn't be too much of an issue, but having persistence with it's contents (aka you can store stuff in it) would likely come at a markup. Considering that it's basically just a (significantly) bigger portable hole, and from what you said, it sounds like it would have it's own air contents, so people inside wouldn't run out of air, I would probably treat it as double the cost of a portable hole for the increased volume, plus the cost of a bottle of air to maintain the atmosphere, so 20k for the first portable hole effect, 30k for the second (extra abilities get x1.5) and 10,875 for the bottle of air effect (again, x1.5 for extra ability), all added up that's about 60,875gp.

[noob]

You can also make a permanent mordenkainen magnificent manor for 182000 gp if you use the continuous item rules.(two times that value if you want it slot-less but often items that have a set up time and can not move easily are considered to have a value as if it was not slot-less)
But then you would have all the stuff that comes with a mordenkainen magnificent manor including a volume of 39 10-ft. cubes, the ability to allow only people you designate to enter the mansion,a door that becomes invisible when you enter,protection against intrusion(except by entering by the portal),some invisible servitors and an environment that might be hostile to you(for example if you are a fish).

[Bronk]

First problem, that specifies that it opens on floors (unlike the Portable Hole), suggesting it won't work on a wall.

It states it works like a portable hole, which works in any orientation. If anything, it says that it has a smaller opening when on the ground, nothing about the normal sized opening.

Second problem, it is a relic tied to a deity, which could cause complications.

You'd have to jump through hoops to use the relic power, which is to open it quickly as a trap, that's all. That power isn't needed.

Third problem, given the RAW rule of what happens when a bag of holding and a portable hole interact, there are possible issues with using multiple of either Enveloping Pit or Portable Hole to create a multi-story extra-dimensional structure, depending on how the DM rules it.

The only RAW interaction is between a portable hole and a bag of holding. There are no bags of holding in this scenario.

[legomaster00156]

It seems to me that you could basically consider this a super-charged Bag of Holding. With a Type IV BoH costing 10,000 gp, I'd put this item at around 15,000-20,000 gp - there are diminishing returns on having even more space for your stuff, and the ability to have distinct rooms and oxygen inside is hardly a game-breaker.

[Quarian Rex]

Well, the closest existing item to what you want is a Portable Hole. That has @280 cubic feet of space and is comparable to the space inside a Bag of Holding IV (250 cubic feet) but with the weight limitation removed (which seems to have essentially doubled the price). The space you're talking about is @95 times the space of the Portable Hole. Looking at the volume to cost of the Bag of Holding I to Bag of Holding IV it looks like you would need @ a Bag of Holding XI to get 30,720 cubic ft. which is just a little over the 27,000 needed for a 30'x30'x30' cube.

A theoretical Bag of Holding XI should cost 27,500 gp, double that to 55,000 gp to remove the weight limit as per Portable Hole, add in the cost of a Bottle of Air 7,250 x 1.5 for additional effect on the same item (10,875) and you have a total cost of @65,875 gp. Perhaps throw on an extra 10% charge to remove the Bag of Holding/Portable Hole astral asplode-y problem, and I think that the door instead of a hole can just be considered a fluff change at no cost.

Total cost for the item 72,462.5 gp. That's being fairly conservative in a way that I don't think any GM would think is exploiting the magic item creation rules.

Hope that is helpful.