A new simple model for sedimentary basin formation, which combines both stretching and phase transitions occurring together in the lithosphere, is proposed, and two important aspects in sedimentary basin formation are addressed: (1) the explanation of the domal uplift preceding rifting, and (2) the ability of the model to explain the contradiction between a small amount of crustal stretching and the large amount of thermal subsidence without the need to invoke differential stretching between the crust and mantle. Relationships for synrift initial and thermal post-rift subsidence are derived. The model predicts that extension of continental lithosphere could produce synrifting uplift up to a threshold amount of extension and then subsidence for greater extension. Three dimensionless parameters quantify the contribution of the phase transition to the subsidence history: (1) the Clapeyron slope normalized to the initial geothermal gradient, (2) the ratio of depth of phase transition to the thickness of the lithosphere, and (3) the ratio of density effect due to phase transition to the change in density due to thermal expansion. An evaluation of these parameters has been carried out by using thermodynamical data for the garnet-spinel transition for upper mantle rocks. Our simple model shows that by including the effects of lithospheric phase transition in existing stretching models of sedimentary basin formation we can obtain an extra significant parameter controlling the subsidence and the entire extensional tectonics of such basins.