In numerical modelling, far from regions of large gradients, the resolved scales contain most of the energy of turbulent motion whilst on subgrid scales (SGS), motions are less energetic. However, SGS contribution becomes larger than the resolved part near the surface. Recent studies have shown that near-surface turbulence anisotropy has a dramatic effect on the mixing length to be used in SGS models which are generally derived for free-stream isotropic turbulence or use the standard Prandtl mixing length $kappa$z. Using the flux measurements collected at the SIRTA observatory located near Paris (France), this paper shows that a SGS model suitable for both the surface layer and free-stream turbulence must discard the Prandtl mixing length and account for near-surface turbulence anisotropy. It also shows that such an SGS scheme, which is suited for all numerical models of the atmosphere, improves the representation of planetary boundary layer processes in most stability conditions.