Mae Llywodraeth y DU wedi cynnig nifer o ddulliau o addasu’r grant bloc yn yr Alban a oedd yn groes i egwyddor “no detriment”. O gymhwyso methodoleg o'r fath i’r ddwy dreth ddatganoledig yng Nghymru, byddai goblygiadau sylweddol ar gyfer buddsoddi mewn gwasanaethau cyhoeddus yng Nghymru yn y dyfodol. Ni fyddai Llywodraeth Cymru yn gallu derbyn y telerau hynny ac nid dyma oedd sylfaen cytundeb y Cynulliad mewn perthynas â datganoli Treth Dir y Dreth Stamp a’r Dreth Dirlenwi i Gymru.
The UK Government has proposed a number of methods for adjusting the block grant in Scotland that were contrary to the no detriment principle. Those methods, if applied to the devolved taxes in Wales would have significant implications for our future investment in public services in Wales. The Welsh Government would not be able to accept those terms and this was not the basis upon which the Assembly agreed the devolution of Stamp Duty Land Tax and Landfill Tax to Wales.