hidden rooms

are hidden rooms always 1 sqaure deep in a wall? I have been digging the first square of areas where they would fit, could they be deeper in the wall than that?

They can be deeper in the wall than one square.

From what I've seen there's 3 types of secret room:

1. Obvious secret room. This is a room-within-a-room, like that one that appears a few floors into the Gloomy Caves.
2. Less obvious secret room. This one is one or two tiles deep into a wall and can be identified by checking for suspiciously square shaped gaps between rooms on the map once you've explored the whole floor.
3. Super secret room. This one can appear in the middle of nowhere, far away from any other rooms. You're unlikely to find this one unless you're specifically looking for it with scrolls of detection, or by being teleported into it by accident.

I think the less obvious secret room can still be hidden deeply, if it's on a level with big gaps like the minefield level.

It'd be cool if there were puzzle rooms to break into.

Is there a predetermined number of secret roons? ie 1 obvious 1 not so obvious amd 1 super secret?

Also can they only appear in the GC or cam there be secret rooms in the UT?

Yes, as of now, all special rooms are predetermined and always exist in each game, only their location is randomized.

I'd be all for having chance-based generation of special rooms, because that would allow for hundreds of hand-made rooms with special contents, only a handful chosen for each game. Much more variety, much more fun.