Over the past several years, non-ablative femtosecond laser exposure with spatially overlapping (i.e., spatially cumulative) pulses has emerged as a key process in three-dimensional writing of patterns in bulk of dielectric substrates. When temporally non-cumulative and combined with post-processing steps, this process defines a novel manufacturing technique in fused silica, finding uses in a broad number of applications, including-but not limited to-micromechanics, integrated optics, microelectronics, microfluidics, information storage, and combinations of these fields for novel integrated sensing applications. For fused silica, evidence has shown that there is a pulse-length duration threshold around 200fs, marking the boundary between two radically different characteristic material modification regimes, each leading to a specific application. Pulse widths below 200fs lead to localized densification, enabling the direct-write of optical waveguides, while pulse widths above this value produce self-organized nanostructures causing a localized volume expansion and enhanced etching susceptibility to various chemicals. Here, we focus our attention on the regime below 200fs, using low repetition rates and temporally non-cumulative pulses. In particular, we use very short pulses, i.e., in the range of 30fs-a regime as yet unexplored from the viewpoint of spatially cumulative modifications. Our goal is to understand how structural modifications obtained by overlapping pulses evolve with varying pulse overlap, and how shorter pulse duration may correlate with higher material densification. This knowledge is particularly important for the next generation of photonics devices, where increasing the level of laser-induced densification is a key factor for high-density photonic integration.