Parasitic architecture allows the creation of flexible structures that feed off existing infrastructure. Additionally, self-organised models that grow in response to environmental forces and adapt to their context introduce new ways for intervening in architectural design. This paper investigates the properties of self-organised parasitic structures that evolve by creating aggregation forms in the context of simulated structural environments. The growth process of the parasitic structures is inspired by the fungal colonies and is based on the rules of diffusion-limited aggregation (DLA) extended to support real-time force analysis and aggregation of space-filling geometry. The results of the simulations demonstrate that the developed diffusion-limited aggregation of truncated octahedrons is capable of providing self-sustained structures able to adapt in environments with different spatial limitations.