  Let $G$ be a countable group and $k$ a positive integer, we show that the
$L^2$-Betti numbers of the group $G$ vanish up to degree $k$ provided that
there is some infinite index subgroup $H$ with finite $k$th $L^2$-Betti number
containing a normal subgroup of $G$ whose $L^2$-Betti numbers are all zero
below degree $k$. This generalizes previous criteria of both Sauer and Thom,
and Peterson and Thom. In addition, we exhibit a purely algebraic proof of a
well-known theorem of Gaboriau concerning the first $L^2$-Betti number which
was requested by Bourdon, Martin and Valette. Finally, we provide evidence of a
positive answer for a question posted by Hillman that wonders whether the above
statement holds for $k = 1$ and $H$ containing a subnormal subgroup instead.
