We have studied the built-in voltage in heterostructures composed of ferroelectric PbTiO3 and dielectric spacers of SrTiO3, grown on SrTiO3 substrates with SrRuO3 bottom electrodes. We used piezoresponse force microscopy to determine the domain structure of the samples and perform local switching experiments. By increasing the thickness of the SrTiO3 spacer layers, we were able to tune the polarization configuration in the PbTiO3 films by gradually going from a monodomain configuration to a polydomain one. The built-in voltage was found to be linked to the choice of electrodes and to the thickness of the spacer layer. The stability of artificially poled regions was also affected, with a faster back-switching for the samples with the thicker SrTiO3 spacers. We have additionally compared samples with and without a top SrRuO3 electrode, showing that a top electrode layer results in a stronger built-in voltage, responsible for a preferential monodomain configuration and a very fast back-switching of artificially poled regions.