  In this note we investigate the asymptotic behaviour of the $s$-numbers of
the resolvent difference of two generalized self-adjoint, maximal dissipative
or maximal accumulative Robin Laplacians on a bounded domain $\Omega$ with
smooth boundary $\partial\Omega$. For this we apply the recently introduced
abstract notion of quasi boundary triples and Weyl functions from extension
theory of symmetric operators together with Krein type resolvent formulae and
well-known eigenvalue asymptotics of the Laplace-Beltrami operator on
$\partial\Omega$. It will be shown that the resolvent difference of two
generalized Robin Laplacians belongs to the Schatten-von Neumann class of any
order $p$ for which $p>(dim\Omega-1)/3$. Moreover, we also give a simple
sufficient condition for the resolvent difference of two generalized Robin
Laplacians to belong to a Schatten-von Neumann class of arbitrary small order.
Our results extend and complement classical theorems due to M.Sh.Birman on
Schatten-von Neumann properties of the resolvent differences of Dirichlet,
Neumann and self-adjoint Robin Laplacians.
