In this work, we present eXtendible Heterogeneous Energy-Efficient Platform (X-HEEP), a configurable and extendible single-core RISC-V-based ultra-low-power microcontroller. X-HEEP can be used standalone as a low-cost microcontroller, or it can be integrated into existing platforms to act like a peripheral subsystem, or it can be extended and customized with external peripherals and accelerators.