Bydd y Bil un pwrpas sengl hwn yn ei gwneud yn bosibl inni gyflwyno newidiadau i Ddeddfau Trethi Cymru drwy reoliadau, os bydd Gweinidogion Cymru o’r farn bod y newidiadau’n angenrheidiol neu’n briodol, ac os bydd angen i’r newidiadau gael effaith ar unwaith neu’n fuan ar ôl cael eu cyflwyno. Caniateir y newidiadau er mwyn ymateb i nifer o amgylchiadau allanol:sicrhau nad yw trethi datganoledig Cymru yn cael eu gorfodi lle na fyddai hynny’n gydnaws ag unrhyw rwymedigaethau rhyngwladol;amddiffyn yn erbyn osgoi trethi mewn perthynas â threthi datganoledig Cymru;ymateb i newidiadau a wneir gan Lywodraeth y DU i drethi’r DU sy’n ‘rhagflaenwyr’ (hy y rheini lle mae trethi datganoledig cyfatebol) sy’n effeithio ar y swm a delir i Gronfa Gyfunol Cymru, neu a allai effeithio ar y swm hwnnw;ymateb i benderfyniadau’r llysoedd neu’r tribiwnlysoedd sy’n effeithio ar sut mae Deddfau Trethi Cymru, neu unrhyw reoliadau a wneir o danynt, yn gweithredu, neu a allai effeithio ar sut y maent yn gweithredu.
This single purpose Bill will enable changes to be made to the Welsh Tax Acts by regulations where Welsh Ministers consider that such changes are necessary or appropriate and where they are required to have effect immediately or shortly thereafter.   Those changes will be permitted in order to respond to a number of external circumstances:to ensure the devolved Welsh taxes are not imposed where to do so would be incompatible with any international obligations;to protect against tax avoidance in relation to devolved Welsh taxes;to respond to changes made by the UK government to ‘predecessor’ UK taxes (that is, those where there are equivalent devolved taxes) which affect, or may affect, the amount paid into the Welsh Consolidated Fund, andto respond to decisions of the courts or tribunals which affect or may affect the operation of the Welsh Tax Acts, or any regulations made under them.