The addition of hydrogen derived from renewable power to the natural gas network is being promoted as a viable means of storing excess wind and solar energy. However, the changes in combustion properties of the natural gas upon hydrogen addition can impact the performance of the end-use equipment connected to the gas grid. We assess the changes in Safety and fitness for purpose of domestic natural gas appliances when supplied with natural gas/hydrogen mixtures. Upon hydrogen addition, the fitness-for-purpose limits are governed by changes in thermal input caused by changes in Wobbe Index, while the changes in risk of flashback are used to quantify the safety aspects. A method is introduced to assess changes in the propensity for flashback, using the computed laminar burning velocity and accounting for changes in equivalence ratio caused by the variations in fuel composition. The computations are seen to reflect the experimental behavior of Bunsen flames regarding flashback upon hydrogen addition to natural gas accurately. Comparing the changes in Wobbe Index and variations in burning velocity with those experienced by domestic appliances subjected to the range of natural gases ordinarily distributed to the end user provides an unambiguous and internally consistent method to assess changes in essential performance upon hydrogen addition. Thus, limits on hydrogen addition can be derived to maintain the current level of safety and fitness for purpose without the necessity of large-scale appliance testing. The results show that the maximum fraction of hydrogen in natural gas that maintains appliance performance depends on the composition of the natural gas to which the hydrogen is added. For fuel-rich premixed appliances, e.g., cooking burners, the maximum hydrogen admixture is seen to be limited by flashback, while loss of thermal input determines the maximum hydrogen fraction in modem lean-premixed appliances. The method is illustrated using a fictitious distribution band, but it can be applied to any regional or national situation. The method presented can be used to define efficient strategic roadmaps and provides essential knowledge for grid management assessments, aimed at introducing hydrogen into the natural gas infrastructure while maintaining performance for the end user.