The new Structure POwder DIffractometer (SPODI), a project of the Technische Universitat Darmstadt and the Ludwig-Maximilians-Universitat Munchen, is currently built up at the new FRM-II reactor in Garching near Munchen. This article describes the concept and the new technical features of the instrument. To optimise all optical components Monte Carlo simulations have been carried out of the whole beam path beginning at the core via the neutron guide system, monochromator, sample collimator, standard sample, detector collimator, up to the detector. Detailed comparisons of various physical parameters like coating and shape of supermirror guides, different detector heights or monochromator angles are discussed. The installation of a detector system with 80 single He3 detectors which are position sensitive in vertical direction opens further applications for the data evaluation. The neutron guide system effectively increases the intensity, but results in a complicated divergence profile at the monochromator. It requires a positioning unit of the monochromator crystals with very high accuracy. The higher monochromator take-off angle of 155 degrees in comparison to other instruments significantly improves the resolution at higher 2 theta angles. Improvements of the detector collimators allow a reduction of the sidewall thickness to increase the number of collimators by 25% by keeping a height of 300 mm.