  Let $n\geq 2$ and $1\leq q<p<\fz$. We prove that if $\Omega\subset\mathbb
R^n$ is a Sobolev $(p, q)$-extension domain, with additional capacitory
restrictions on boundary in the case $q\leq n-1$, $n>2$, then
$|\partial\Omega|=0$. In the case $1\leq q<n-1$, we give an example of a
Sobolev $(p,q)$-extension domain with $|\partial\Omega|>0$.
