The prescriptive approach of the Moroccan Building Thermal Regulation (RTCM 2015) requires thermal insulation of buildings’ slab-on-ground in almost all climatic zones of Morocco. This work aims at demonstrating that this requirement is an unnecessarily expensive constraint for most Moroccan climatic zones where the buildings’ slab-on-ground temperatures below 19 °C or above 26 °C account for only 8.6% and 22% of the cold and hot semester days, respectively. This work shows also that the slab-on-ground of a building is subject to (i) a slow mono-dimensional vertical heat transfer as the outdoor air temperature long-term extrema are delayed for around 22 days and (ii) a faster bi-dimensional horizontal heat transfer as the outdoor air temperature singularities are delayed for about 2 days by 5 m from the edge of the building. To limit this undesired lateral heat transfer, the authors recommend lateral insulation of the building’s foundations over 50 cm depth. In addition, building thermal simulation software needs improvements regarding the site-specific models of the seasonal variation for buildings’ slab-on-ground temperature. A solution is proposed to obtain these variations directly from those of the outdoor air temperature. It is shown that the slab-on-ground surface temperature varies almost like the undisturbed underground temperature at 1.6 m depth for the studied case.