I think I can see the inspector's issue.
When you build a house, you have to do a load calculation. What he's saying is that you also need to do this when you add to the house. In this calculation, there would be two questions to answer:
First, is the service to the house large enough to support the addition? Does adding 300 sq. ft. overload your service?
The second is whether you have enough power for that addition. That is, using the 3 watts/sq. ft. rule and accounting for fixed equipment (like an air conditioner), is the circuit large enough to feed it all?
Let's look at an imaginary 15 amp 120v circuit:
3 watts / sq. ft.
15 amps x 120 volts = 1800 watts
1800 / 3 = 600 sq. ft.
That circuit can serve a maximum of 600 sq. ft.
