Let $K$ be the quotient field of a Dedekind domain $R$. We characterize the $R$-orders $\Lambda$ in a separable $K$-algebra for which every $R$-projective $\Lambda$-module decomposes into $\Lambda$-lattices. Butler, Campbell and Kovács have recently shown that the latter holds for the integral group ring of a cyclic group of prime order, as well as for lattice-finite orders over a complete discrete valuation domain.