Mae'n bwysig bod yr asesiad o'r effaith ar dreftadaeth yn gymesur ag arwyddocâd yr ased hanesyddol ac â maint y newid arfaethedig. Paratowyd dogfen ganllawiau, Asesiadau o’r Effaith ar Dreftadaeth yng Nghymru, i helpu perchenogion, meddianwyr ac asiantiaid  i ddeall y broses asesu ac i baratoi datganiad o'r effaith ar dreftadaeth. Mae'r ddogfen ganllawiau honno'n rhan o'r ymgynghoriad hefyd.
It is important that a heritage impact assessment is proportionate both to the significance of the historic asset and to the degree of proposed change. A guidance document, Heritage Impact Assessment in Wales, has been produced to help owners, occupiers and agents understand the assessment process and prepare a heritage impact statement. This guidance document is also part of the consultation.