Smallish interior door

Its a bit of a weird setup in my house but I am going to knock through from my extension into my utility room to create a nice new doorway.
However space is tight. The opening will probably allow for a door which will measure 25/26 innches wide.
However all of the doors I see advertised for sale are either 27inches or 30 inches wide. Can I just buy one and plane it down or is it best to buy a specially made one? Are there any regs regarding the size of a door and also which way it opens i.e. into the main room or out of it?
Any help/advice would be greatly received.
Chris

27" is an accepted minimum for a doorway. any less will render the room difficult to use, as you will not be able to get larger stuff in or out.

you may need a joiner to build you such a door.