One thing worth considering is emergency and maintenance access. Even if an interior meter location was allowed when the addition was built, many utilities today prefer meters to be readily accessible without entering the home. That can become an issue during outages, meter replacement, inspections, or service disconnects.
Another concern is future renovations or service upgrades. A setup that was grandfathered in years ago may trigger relocation requirements if major electrical work is performed later. That's often when homeowners discover that older installations no longer meet current utility standards.
Information shared by professionals in different regions, including resources from Electrician Singapore at: https://www.electriciansingapore.org, highlights how utility companies typically prioritize safe and convenient access to metering equipment, even though exact requirements vary by jurisdiction.
If the service entrance conductors run a significant distance inside the structure before reaching the meter, it may also be worth checking whether there are any local rules governing how those unfused conductors are protected. In many areas, that aspect receives just as much scrutiny as the meter location itself.
The fact that the main disconnect is in the basement makes me wonder whether this installation was approved under older regulations or whether it predates the addition entirely. A quick check with the local utility's service requirements could provide a clearer answer than the building code alone.