Active optical components are essential building blocks for a wide variety of applications such as optical communications, microscopy, and illumination systems. Reconfigurable metasurfaces, which consist of arrays of sub-wavelength meta-atoms, can be engineered to uniquely realize compact and multifunctional optical elements, enabling light-polarization dynamic-control as well as beam steering, focusing or zooming. Varifocal metalenses, in particular, have attracted increasing interests. Yet, going beyond mechanical modulation schemes to realize ultra-thin devices with fast modulation remains challenging due to the complex phase and phase-delay profiles involved. Recently, thermooptical effects in dielectric nanostructures have emerged as a promising solution to tune their optical resonances, offering unexplored opportunities for ultra-thin reconfigurable metalenses, in particular silicon based ones. In this work, we report a proof-of-concept design of an ultrathin (300 nm thick) and thermo-optically reconfigurable silicon metalens operating in the visible regime (632 nm). Importantly, we demonstrate that, using thermo-optical effects, it is possible to achieve continuous modulation of the focal-length at a fixed wavelength. In particular, operating under right-circularly polarized light, our metalens exhibits a linear focal shift from 165 mu m at 20 degrees C to 135 mu m at 260 degrees C, exceeding the lens focal depth. The average conversion efficiency of the lens is 26%, close to mechanically modulated devices, while its Strehl ratio is 0.99, confirming a diffraction-limited performance. Concurrently, in this work we report an automatized methodology to design a reconfigurable metalens, compute its layout and verify the expected performance. Overall, we envision that, by further optimization of the optical response of individual meta-atoms with machine-learning algorithms, thermally-reconfigurable silicon metalenses will emerge as a viable, chip-compatible solution to realize ultrathin varifocal lenses.