If agreement cannot be reached within 21 days of the request for a variation, the taxpayer or the WRA can apply to the Tribunal for a determination on the variation.
Os na ellir dod i gytundeb cyn pen 21 diwrnod ar ôl y cais am amrywiad, gall y trethdalwr neu ACC wneud cais i'r Tribiwnlys am benderfyniad ar yr amrywiad.