Let $\Omega  \subset \mathbb{R}^n$ be open and bounded with a sufficiently smooth boundary. Let $L$ be a second order differential operator with variable coefficients, given by
$$Lu = \partial_i(a^{ij}\partial_ju) + b^i\partial_ku + c u.$$
For an arbitrary $f$, consider the boundary value problem posed in $\Omega$ 
$$
\begin{cases} Lu = f \\
\text{with boundary conditions}\\
\end{cases}
$$
The well-posedness of a weak solution and its regularity is extensively studied.

For instance, in [Evan's PDEs][1] we find for Dirichlet boundary conditions

> THEOREM 5 (Higher boundary regularity). Let $m$ be a nonnegative
> integer, and assume $ a^{i j}, b^i, c \in C^{m+1}(\bar{\Omega}) \quad(i,
> j=1, \ldots, n) $, and $ \partial \Omega \text { is } C^{m+2} $, and $ f \in H^m(\Omega) . $
> Suppose that $u \in H_0^1(\Omega)$ is a weak solution of the boundary-value problem.
> Then $$ u \in H^{m+2}(\Omega), $$ and we have the estimate $$
 \|u\|_{H^{m+2}(\Omega)} \leq C\left(\|f\|_{H^m(\Omega)}+\|u\|_{L^2(\Omega)}\right),
$$ the constant $C$ depending only on $m, \Omega$ and the coefficients of
> $L$.

In [Grisvard's Elliptic Problems in Nonsmooth Domains][2] we find Theorem 2.5.1.1 which gives the same result for more general boundary conditions but with $a^{ij} \in C^{m,1}(\overline{\Omega}), b^i \in C^{m,1}(\overline{\Omega})$ and $ \partial \Omega \text { is } C^{m+1,1}.$

My question is, what is the most relaxed conditions one can assume on the coefficients $a^{ij},b^i,c$, and $f$, and still get a similar regularity in Sobolev spaces as given in the previous theorems? The boundary can be assumed to be sufficiently smooth! 

Providing references for further reading is much appreciated.

  [1]: https://bookstore.ams.org/gsm-19-r/
  [2]: https://epubs.siam.org/doi/book/10.1137/1.9781611972030