We present a novel way to synthesise the radio emission from extensive air showers. It is a hybrid approach which uses a single microscopic Monte-Carlo simulation to generate the radio emission from a shower with a different longitudinal evolution, primary particle type and energy. The method employs semi-analytical relations which only depend on the shower parameters to transform the radio signal in the simulated antennas. We apply this method to vertical air showers with energies ranging from $10^{17} \; \text{eV}$ to $10^{19} \; \text{eV}$ and compare the results with CoREAS using two different metrics. In order to gauge the performance over our simulation set, we subsequently use every shower in the set as a template to synthesise the emission from the other showers. Depending on the scoring metric, template synthesis reconstructs the radio emission with an accuracy of 5 to 10%.