As basic income (BI) has ascended the policy agenda, so proposals have come under increasing scrutiny for their affordability and adequacy for meeting need. One common objection to BI has been that it is impossible to design a scheme that simultaneously conforms to these two criteria. In this article, I develop a conceptual framework for analysing the trade-offs that afflict BI policy design. I suggest that while the idea of a policy dilemma between affordability and adequacy does indeed afflict ‘full’ BI schemes, it is possible to design an affordable and adequate ‘partial’ BI scheme. However, this comes at the cost of (at least partly) forfeiting some key advantages that motivate interest in BI in the first place, since these only arise as a consequence of the elimination of means testing and related conditionality from the welfare system. Thus, BI proponents face a three-way trade-off in policy design between affordability, adequacy, and securing the full advantages of BI as a radical simplification of existing welfare policy. The trilemma is illustrated with reference to original microsimulation evidence for the UK, which demonstrates that at most two of the three criteria can be achieved in a single scheme.