Abstract Land‐surface and atmospheric models often represent subgrid‐scale variability using a single set of effective properties. Estimating these equivalent properties is critical for predicting land‐atmosphere exchanges accurately, but challenging when materials with distinct radiative and thermal characteristics coexist, particularly in urban environments. Here, a formulation for the equivalent thermal properties of horizontally heterogeneous land surfaces is derived from surface energy balance and subsurface heat‐conduction principles under diurnal forcing. The equivalent heat capacity is the area‐weighted mean of individual material capacities, while the equivalent diffusivity reproduces the correct average depth of diurnal temperature penetration. For a representative composition of six materials, simulations yield mean absolute errors of 0.34°C and 4.32 W m−2 ${mathrm{m} }^{-2}$, which remain below 0.90°C and 11.4 W m−2 ${mathrm{m} }^{-2}$ across 50 area‐ratio configurations. The proposed formulation generally outperforms established approaches and is especially useful over surfaces strongly modified by anthropogenic activities.