A football club should always have a ground big enough to cater for its catchment and achieve its full potential.
Having just 6300 home seats and able to cater for just 1400 visiting make Sixfields not fit for purpose when you consider we have a 300,000 people living within 10 miles.
So is your belief that Sixfields is a non league ground based predominantly on the capacity and not on appearance?
The reason I asked for other examples of grounds with terracing above the seats is because I cant see how it would make Sixfields look anything other than non league or an embarrassment with that set up.