Planar Periodic Surface Lattice (PSL) structures based on thin, subwavelength substrates have been studied experimentally and numerically. Coupled eigenmode resonances composed of partial volume and surface modes are observed for PSLs with lattice periodicities of 1.50 mm and 1.62 mm etched onto thin copper-backed, substrates. We show that the copper backing is essential for mode-selection in a multi-moded structure and demonstrate good agreement between the experimental results and coupled dispersion plots calculated using CST Microwave Studio. For the first time, evidence of a coupled eigenmode in a metadielectric PSL is presented. It is shown that metadielectric PSLs can support coupled resonances over a narrow bandwidth and are relevant to the innovation of tunable filters, absorbers and sources. Concepts discussed in this work are valid across the frequency spectrum from optical to THz and mm-wave frequencies and are fundamental to the innovation of novel mm-wave-THz sources as well as highly efficient solar cells, diagnostic instruments and antennae.