First order phase transitions in cosmology are usually assumed to proceed via bubble nucleation in homogeneous spacetime. The presence of impurities, or seeds, in the early Universe can however alter this picture by providing an additional channel for the false vacuum decay with exponentially enhanced tunneling probability. In this talk we will show how a seeded electroweak phase transition is realized already in the simplest extension of the SM including a scalar singlet with Z_2 symmetry (xSM), where the role of impurities is played by the Z_2 domain walls. We will discuss the various methods that allow us to evaluate the seeded nucleation rate, and show that this is generically faster than the homogeneous process. In this case gravitational waves are then produced by colliding walls rather than ordinary bubbles, with the duration of the phase transition set by the distance between the walls. For defects close to the scaling regime the resulting gravitational wave signal is generically amplified and shifted to lower frequencies.