You're describing a common sort of "creep" in building design.
The 'creep' occurs when a porch with a simple handrail first gets the spaces under the rails closed off .... then screens put up all around .... then glazing added. As this takes place, the porch begins to be used more ... from summer BBQ's, to a play area for the youngsters, to overflow seating for holiday meals or sheltered 'outside' smoking, to suddenly being used as a full-time, year-round room.
To a certain extent, the same issues can arise in mudrooms.
I think the key is the term 'habitable.' Such an enclosed area in the front of the house is unlikely to ever be a proper room - not with entry traffic constantly passing through. When the same happens to a patio in the rear, or a balcony, the picture might change.
That still leaves the matter of enforcement. With such an incremental series of changes, I don't see where permits are ever likely to be requested.
In a similar manner, I suspect that very few basement remodels are aver inspected. I think we have to recognize the limits of inspections, and accept that this sort of thing simply can't be regulated.
Otherwise ... if asked for an honest opinion, I'd look at the specific layout, and try to imagine it's possible uses. Even if they're calling it an office,' for example, circumstances might lead me to consider it as a bedroom.
