Laser guide stars (LGSs) aim at increasing the sky coverage of adaptive optics (AO) as this is highly restricted when using only natural guide stars. With such three-dimensional extended objects, spot elongation may limit the measurement accuracy of wavefronts. We evaluate the effect of differential focal anisoplanatism, induced solely by the longitudinal extension of a side-launched LGS, on the slope measurements performed by a Shack–Hartmann for a $\mathrm{40}\mathrm{m}$ class telescope. We also take this effect into account in the wavefront reconstruction and derive estimations of the resulting wavefront error in a multi-LGS AO system. We find an error of $100\mathrm{nm}$ in the worst case at the subaperture level and a small error of the order of $10\mathrm{nm}$ for six LGSs after wavefront reconstruction.