I would base it on the egress or lack of egress more likely.
Exactly. The question is whether egress means is required for a given room.
For example: The homebuilder/owner doesn't want to pay the extra cost of a large-enough window and the window-well and climbing-out means necessary, and the building inspector says he must, because the room can be considered useable as a bedroom.
If the room qualifies as a bedroom, he must pay for the extra costs required. So, there must be something beyond the inspector's personal opinions when determining what must be included in the construction of the house. Smoke detectors enter the debate, too.
Added: Once the house is built, there's nothing to prevent the parents from moving Junior into the basement room whether it's a legal bedroom or not.