Fajr is "qaza" once one does not pray it in its time-limit ie before sunrise. After sunrise, it is qaza. Before mid-day ("zawal"), fajr salah should be prayed with 2 rak'ah sunnah + 2 rak'ah fajr. After zawal, only 2 fards should be prayed if fajr is still not performed. Doing Qaza of fajr must of course not be made a habit. Wallahu a'lam. Wassalam.