In the traditional finite difference time domain (FDTD) method, the studied structure is modeled as elementary cells, which sizes have to be small enough to get close to the reality. Then, this discretization is applied to the whole calculation volume, even though some zones do not need such fine discretization. Consequently, this numerical tool requires an important calculation capacity to increase the precision of the grid mesh. For very large dimension structures, computer limitations no longer allow a sufficiently accurate discretization. The aim of this article is to discuss some methods which allow the EM study of large structures containing elements of very unequal size as compared to the wavelengths used, while preserving an acceptable calculation cost. This approach combines finite differences of variable precision and different-sized cells in the same calculation doMayn. It allows a fine discretization of small structures and a rough discretization of the other elements of the EM problem. This spatial subgridding gives a very original possibility of undertaking EM zooms on some specific parts of the calculation doMayn thus allowing a better modelling of the studied structure while preserving good accuracy and an acceptable cost.