The design of nanostructured materials with specific physical properties is generally pursued by tuning nanoparticle size, concentration, or surface passivation. An important step forward is to realize “active” systems where nanoparticles are vehicles for controlling, in situ, some specific, tuneable features of a responsive functional material. In this perspective, this work focuses on the rational design of a nanostructured glass with electrically tuneable dielectric function obtained by injection and accumulation of charge on embedded conductive nanocrystals. This enables electrically controlled switching of semiconducting nanophases to charged polarisable states to be achieved, which could lead to smart, field-enhancement applications in nanophotonics and plasmonics. Here, it is shown that such response switching can be obtained if a percolating charge-transport mechanism is activated through a disordered tree-like network, as is demonstrated to be possible in SiO2 films where suitable dispersions of SnO2 nanocrystals, with conductive interfaces, are obtained as a result of a new synthesis strategy.