  The zero sets of harmonic polynomials play a crucial role in the study of the
free boundary regularity problem for harmonic measure. In order to understand
the fine structure of these free boundaries a detailed study of the singular
points of these zero sets is required. In this paper we study how "degree $k$
points" sit inside zero sets of harmonic polynomials in $\mathbb R^n$ of degree
$d$ (for all $n\geq 2$ and $1\leq k\leq d$) and inside sets that admit
arbitrarily good local approximations by zero sets of harmonic polynomials. We
obtain a general structure theorem for the latter type of sets, including sharp
Hausdorff and Minkowski dimension estimates on the singular set of "degree $k$
points" ($k\geq 2$) without proving uniqueness of blowups or aid of PDE methods
such as monotonicity formulas. In addition, we show that in the presence of a
certain topological separation condition, the sharp dimension estimates improve
and depend on the parity of $k$. An application is given to the two-phase free
boundary regularity problem for harmonic measure below the continuous threshold
introduced by Kenig and Toro.
