Metasurfaces, typically constructed from spatial arrangements of localized building blocks, can enhance light–matter interactions through local field enhancement or by coherent coupling to extended photonic modes. Recent works have explored how guided mode resonances influence the performance of nonlinear metasurfaces. Here we investigate the modal impact on difference-frequency generation in a waveguide-coupled metasurface platform. The system is constructed from gold split-ring resonators on a high-index TiO₂ waveguide. We find that a symmetric configuration of the metasurface’s localized modes and the extended waveguide modes lead to a modest enhancement of the downconversion process. However, when the mirror symmetry of the localized modes with respect to the guided mode propagation breaks, it introduces external chirality. This enables coupling to a higher quality mode, resulting in a 70-fold enhancement of the difference-frequency generation. The capacity to manipulate the nonlocal modes through the design offers broader control over the interaction and new avenues to tailor the nonlinear processes.