The extension to the Bankside gallery, dubbed the 'misshapen pyramid' by critics (below), went in for planning last October, causing uproar among some heritage groups.
Pierre de Meuron (above) told the AJ that the extension had to happen, and an access point in the south side was the most logical step.
He said: 'Tate Modern had a triggering element, in terms of people visiting museums. I don't think anyone realised how successful it would be when we were doing it.
'It was clear, as there was already access from the north and the west, that there needed to be something on the south side.'
But de Meuron insisted that the extension would not be a detriment to the former Bankside Power Station.
'There had to be more gallery space after the success of the Tate Modern,' he said. 'But we were careful not to break the spirit of the existing building - not to break the industrial spirit.
'But now it is a new building so this is the challenge we have to face, how to keep the spirit of the building while creating a new one.'
Herzog and de Meuron last night received the RIBA Gold Medal in a ceremony at Portland Place.