It's really a call that ought to have been made when the plans were submitted for approval by the building department.
Even with a custom home, there is quite a turnover, with most being sold in five years or so. It is very possible that today's "sewing room" will be presented as an additional bedroom at the time of sale. I would look at the room, and ask:
- Is it really big enough for a bed and dresser?;
- Is it located together with the other bedrooms:,
- Is it placed so traffic passes through it?;
- Are there any special features to define its' use?; and,
- Is the argument worth more than the $35 to put in an AFCI breaker?
