The measurement of surface impedance and the absorption/transmission characteristics is well-established for acoustic materials that can be considered locally reactive: Impedance tube-based and in-situ measurement techniques are codified in multiple ISO standards, e.g. ISO 354 and ISO 10534. However, as lateral wave propagation in the material under test becomes significant, locally reactive assumptions fail and the standardized measurement methods become infeasible. The present contribution discusses the measurement of non-locally (or extended) reactive surfaces by estimating its distributed surface impedance function and the consequent numerical prediction of the reflected sound field in a simple planar geometry. The model is validated via the measurement of a resonant panel absorber. Finally, the extension of the absorption coefficient for non-locally reactive surfaces is discussed.