On the height of polynomials that split completely over a fixed number field

Let $K\mathrm{/}\mathbb{Q}$ be a finite extension. We prove that the minimal height of polynomials of degree $n$ of which all roots are in $K^{\times }$ increases exponentially in $n$. We determine the implied constant exactly for totally real $K$ and $K$ equal to $\mathbb{Q}(\sqrt{-1})$ or $\mathbb{Q}(\sqrt{-3})$.