There are a number of different methodologies which each has its advantages and disadvantages, as well as a number of other factors such as changes in the value of land or in building costs.
Ceir nifer o wahanol fethodolegau y mae gan bob un ohonynt ei manteision a'i hanfanteision, yn ogystal â nifer o ffactorau eraill megis newidiadau yng ngwerth y tir neu mewn costau adeiladu.