"Is it a bedroom"? The best answer I can provide my clients is; "let the building inspector decide". I suggest to the client that they obtain a building permit for "a change in use".
In most locations a building permit is REQUIRED for ALTERATIONS and CHANGE IN USE OR OCCUPANCY even if no construction is being done. I present to the owner that the cost of the permit is small in comparison to the increase in the value of the home by having an additional bedroom.
The reason: Unless the appraiser has access to the original building plans and any building permits issued after the original occupancy permit was issued, the appraiser has no evidence of what the approved plans labeled the room (bedroom, den, office etc.).
If a building permit is issued, the building inspector will determine if the room is a bedroom. The building inspector will make sure the room has the correct egress (usually a door and fire window but there are cases where two doors are acceptable), the proper ventilation, a smoke detector if required, meets the septic system requirements if required etc., required by local code to use the room for sleeping. The building inspector should then approve the permit, approve the use of the room for sleeping.
Once the owner has both a copy of the permit application AND the approved permit (if it is approved with no alterations) they now have a bedroom/sleeping room. I also advise them to leave copies for the appraiser.
I also retain a copy in case of a question from another agent or the local Association of Realtors.