A recently developed Debye–Hückel-type ion–ion interaction free energy model based on concentration and temperature-dependent dielectric susceptibility is applied to homogeneous electrolyte bulk regions where it is known that non-constant susceptibility is necessary for a correct description of the heat capacity and the apparent molar volume. The model imposes a thermodynamic compatibility condition that prescribes the temperature dependence of the susceptibility. Here, this condition is successfully tested against experimental data. Contrary to the electrolyte bulk, where in equilibrium the electric field vanishes, the field strength can rise extremely high in charged boundary layers. Therefore, we consider in the layer an explicit polarization free energy contribution instead of the ion–ion interaction energy and a dependence of the susceptibility on the electric field to the model. The strong influence of combined field and concentration dependence of the susceptibility on the double-layer structure with its spatial distribution of ions and the solvent is illustrated. The impact of the field-dependent dielectric saturation on the boundary layer capacitance is studied numerically.