If you have started your tax return, then realise you need new codes added, you will have to delete the draft and re-create once the new codes have been added.
Os na wnaethoch ei hawlio, a’ch bod yn sylweddoli’n ddiweddarach y dylech fod wedi gwneud hynny, gallwch ddiwygio eich ffurflen dreth a hawlio ad-daliad o’r dreth ychwanegol a dalwyd gennych.