Tunable manufacturing using flexible building blocks to bring functionalities while providing long-term stabilization for small-sized nanomaterials occupies a forefront position in nanoscience and nanotechnology. Iron oxide nanoparticles are among the most promising nano-objects, owing to their inherent magnetic properties. The propensity of iron oxide to aggregate constitutes a serious drawback for many of their potential properties that need to keep their size at the nanoscale. Hosting iron oxide in porous supports, intercalation in layered nanostructures or shelling with tunable partners are common approaches used for their stabilization. We herein report the controlled growth of iron oxide using cleverly crafted sol-gel transformable siloxane precursors. These flexible building blocks provide an entry to iron oxide encapsulated in hydrophobic silica, denoted as Fe3O4@PMSiO2, while the use of commercially available TEOS afforded Fe3O4@SiO2. Owing to the presence of PDMS-like segments, the grown Fe3O4 exhibits distinctive features in terms of the crystal size, restricted growth and dispersion in organic solvents compared to native iron oxide and those grown in conventional silica supports. Upon calcination at 500 °C, the size of the crystal expands by 25.4 nm in the case of native Fe3O4 reaching 35.9 nm. Comparatively, marginal expansion was observed using our siloxanes, with the size of those grown in Fe3O4@OMSiO2 and Fe3O4@PMSiO2 being restricted to 14.3 nm and 11.7 nm, because of the stabilization brought by the siloxane layers. Furthermore, Fe3O4@OMSiO2 and Fe3O4@PMSiO2 are fully soluble in apolar heptane and hexane, which convincingly substantiate the hydrophobic nature of the resulting mixed oxide materials.Graphical Abstract