Let $\Omega \subset \mathbb R^n$ be a Lipschitz domain, let $s \in \mathbb R$. Let $W^s(\Omega)$ denote the Sobolev-Slobodeckij space on $\Omega$, and let $H^s(\Omega)$ denote the Bessel-potential spaces on $\Omega$, each for the parameter $p = 2$.
Both spaces are often refered to simply as Sobolev spaces but are not the same in general. Under what set of additional conditions (which may be the empty set), do these spaces coincide, i.e. $W^s = H^s$?
I have not found a resource that explains the situation.