It is believed that the rebuilding work began sometime between 16 February and 2 March.
The site is where North Korea carried out satellite launches in recent years, inviting rounds of sanctions from the UN that saw it as a disguised test of missile technology.
South Korea's JoongAng Ilbo newspaper reported that the country's spy service also gave such an assessment on the site in a private briefing on Tuesday.
JoongAng Ilbo cited unidentified politicians who attended the briefing as saying that National Intelligence Service director Suh Hoon told them that the structures being restored at the launch site include roofs and doors of buildings.
Mr Suh was quoted as saying that the move is seen as a preparation to restart long-range missile test-launches in the event that talks over denuclearisation collapse.
He also reportedly said the rebuilding of some structures was to blow up the launch site more dramatically as a sign of commitment to denuclearisation if the talks had gone well.