Considering the transition towards the hydrogen economy, dependent on hydrogen penetration scenario, the cost of a new hydrogen pipeline infrastructure in Europe may amount to several thousands of billions of EURO's. Therefore, the examination of the potential contribution of the existing natural gas assets is a practical and logical first step. As the physical and chemical properties of hydrogen differ significantly from those of natural gas, it is not at all possible to simply exchange natural gas by hydrogen in the existing infrastructure. In this paper first a brief overview will be given of the NATURALHY-project. Further the focus will be on the impact of added hydrogen on the performance of existing natural gas domestic end user appliances, which is related to the operation of the natural gas grid connecting the different types of appliance. The application of the fundamental insights and carefully designed experiments, comparing the behaviour of gases using justified reference conditions, have been shown to offer essential progress. The Wobbe index limits of the natural gas distributed pose a first limiting factor upon the maximum allowable hydrogen concentration. Constant-Wobbe index and decreasing-Wobbe index options of H2 admixture have been studied. Considering the appliance light back H2 limiting factor for domestic appliances, fuelrich appliances are the critical ones. Also taking into account stationary gas engines, gas turbines, industrial applications and natural gas grid management, it is not yet justified to present statements on what level of hydrogen concentration could be safely allowed in which specific natural gas distribution region. But more clarity has been obtained on combustion safety aspects of existing domestic appliances, on the connection with Wobbe distribution conditions and on the bottlenecks still to be handled.