Heterogeneous materials with tailored, site-specific physical properties, ubiquitously found in biological systems, can reach optimal performances in applications. Compared to homogeneous materials or composite materials with random, disordered microstructures, materials with locally addressable properties achieve higher strength at lower density, or record-high shock absorption properties. A classic example of tailored, heterogeneous materials are functionally graded materials or materials where stiffness variations in bulk are controllable. However, current methods to locally control stiffness in a solid are mostly limited to geometrical variation of architected micro-structures, or to the creation of multi-material composites. Here, a novel approach to program stiffness variations within a single material system is presented. By subsequent exposure of a pre-crosslinked polymeric matrix using two-photon polymerization, arbitrary three-dimensional architectures with locally addressable stiffness variations are fabricated. Furthermore, the extension of the hierarchical level of three-dimensional micro-architectures by property variations in the constituent material is demonstrated. The programmable stiffness variations, in the sub-micron range, are either continuous or sharply confined, with a stiffness increase of up to 200% compared to the pre-crosslinked matrix. This site-specific control of stiffness and internal architecture opens the door for engineering materials with unprecedented mechanical properties.