Planning for retirement and subsequent execution of the plan are difficult, but essential for financial security in old age. To formally analyse the interplay between planning and self-control, I introduce cognitive costs of formulating a plan into the two-system model of impulse control. The resulting possibility of rational inaction in pension choices allows to account for a range of robustly observed behaviours. The model indicates that although automatic enrolment into private pensions raises participation (the `default option effect'), its impact on aggregate savings remains ambiguous as some individuals are `forced' to save, while others become `discouraged' and adhere to possibly low default contribution rates. Due to the same mechanism, automatic enrolment reduces cross-agent variation in wealth accumulation. A simple calibration of the model to the UK data shows that while automatic enrolment can increase aggregate savings, its impact on welfare is limited because age-independent contribution rates are suboptimal from the perspective of life-cycle consumption smoothing. Importantly, the effects on savings and welfare are highly heterogeneous across income categories.