Trap CR: Help me calculate this trap

[EPRock]

I am in the process of Converting Dungeonland from 1st Edition to 3.5.

So far it has not been going too poorly, and actually has been quite interesting and helping me build stat blocks for monsters quicker and faster. I have also been beefing up some of the monsters as my characters are all 13th level.

I need some help in figuring out this trap. I need some help in determining Search DC's, CR, and Spells auras used in the creation of this trap.

The characters enter the Mad Hatters/March Hares home, after three rounds the whole house transforms into a giant caterpillar trapping the characters inside, the house starts spewing forth digestive juices (acid) which deal 1d4 plus an additional d4 damage every level, so it is 2d4 the second round, 3d4 the third round, etc...

After 10 rounds the house reverts back to normal and spits the remains out the back door.

- What is a good Search DC for this trap and where should they have to search (Doorway?)
- What is a good CR for this trap
- What spells would be used to create this trap from the PHB

- I was thinking of the following Spells: Polymorph Any Object, and Permanency.
- Any other suggestions or help for me figguring this out.

 

[dcollins]

You may be assisted by the conversion already in ENWorld's conversion library, here: http://www.enworld.org/modules.php?op=modload&name=Downloads&file=index&req=viewsdownload&sid=4

 

[EPRock]

That conversion does not take into account this bldg

Unfortunately that conversion does not take into account that building. Does not have any specs on it.

I am using some information from that conversion.

 

[MerakSpielman]

Hmmm... that's 55d4 total, or a bell curve between 55 and 220 points of damage, average about 140 points of damage per character. Pretty nasty. The whole house is the trap, yes? I'd say searching any part of the inside or outside of the house might reveal its nature. A pretty high DC, since this is a pretty sophisticated trap, say, DC 30? I'd give it a CR of at least 15 or so, maybe higher, unless there's some way to get out of the trap early, before the final rounds of extreme damage.

 

[EPRock]

How about this?

According to the Designing a Trap section in the DMG it is +1 per 7 points of average damage per round so it works out to be around CR 21 Owch.

Base Magic Trap: CR 1

Average Damage (Calculates the +CR).
1: 2 +0
2: 5 +1
3: 8 +1
4: 10 +1
5: 13 +2
6: 15 +2
7: 18 +3
8: 20 +3
9: 23 +3
10: 25 +4

The characters can also breach a hole in the house by dealing 300 points of damage to the interior of the catepillar to escape.

Search DC 30, Disable Device 30

Trigger: Proximity
Reset Type: Automatic

So the spells needed would be probably the following:
- Polymorph Any Object
- Melfs Acid Arrow
- Permanency
- Alarm

Does this sound right?