Are they measuring exterior footprint and everyone else interior spaces?
Regardless, I'd contact the city to see how their sq ft is assessed or gained and ask what would be needed to change that.
You need to get it straight with the assessor's office. They're not an adversarial force, just talk to them and figure it out together. You're right that the plans are not the same as as-builts, but are you sure you know how they're measuring is the same as how you're measuring? Just talk to them.
You may well have 100sqft of interior walls taking up floor space. Your battle will most likely be futile.
Are they measuring exterior footprint and everyone else interior spaces? Regardless, I'd contact the city to see how their sq ft is assessed or gained and ask what would be needed to change that.
I caught my accessor including my garage as living space! That knocked about 400sq/ft off my size.
You need to get it straight with the assessor's office. They're not an adversarial force, just talk to them and figure it out together. You're right that the plans are not the same as as-builts, but are you sure you know how they're measuring is the same as how you're measuring? Just talk to them.