Underpinning in an existing foundation with piles is commonly used to improve their load-bearing capacity to allow vertical extension works. This study evaluated the effect of the piles’ stiffness on the load-sharing behavior of existing and underpinning piles during the vertical extension of existing buildings. A series of 1 g model tests, all with a scale factor of 1/20, was used to compare the underpinning performance of micropiles of varying stiffness. The performance was evaluated in terms of total settlement reduction and load sharing capacity. The results showed that the underpinning performance improved with the increase of underpinning pile stiffness. The ratio of carried loads of the underpinning pile to the existing pile increased linearly with its increasing stiffness (as expressed as a ratio relative to the stiffness of the existing pile).