A necessary condition for Sobolev extension domains in higher dimensions

We give a necessary condition for a domain to have a bounded extension operator from $L^{1,p}(\mathrm{\Omega })$ to $L^{1,p}({\mathbb{R}}^n)$ for the range $1<p<2$. The condition is given in terms of a power of the distance to the boundary of $\mathrm{\Omega }$ integrated along the measure theoretic boundary of a set of locally finite perimeter and its extension. This generalizes a characterizing curve condition for planar simply connected domains, and a condition for $W^{1,1}$-extensions. We use the necessary condition to give a quantitative version of the curve condition. We also construct an example of an extension domain that is homeomorphic to a ball and has $n$-dimensional boundary.