We study the Dirichlet and Neumann boundary value problems for the Laplacian in a Lipschitz domain Ω, with boundary data in the Besov space B s p,p (∂Ω). The novelty is to identify a way of measuring smoothness for the solution u which allows us to consider the case p<1. This is accomplished by using a Besov-based non-tangential maximal function in place of the classical one. This builds on the works of D. Jerison and C. E. Kenig [J. Funct. Anal. 130, No. 1, 161–219 (1995; Zbl 0832.35034)] where the case p>1 was treated.