  Let $A$ be a left-symmetric (resp. Novikov) algebra, $E$ be a vector space
containing $A$ as a subspace and $V$ be a complement of $A$ in $E$.The
extending structures problem which asks for the classification of all
left-symmetric (resp. Novikov) algebra structures on $E$ such that $A$ is a
subalgebra of $E$ is studied. In this paper, the definition of the unified
product of left-symmetric (resp. Novikov) algebras is introduced. It is shown
that there exists a left-symmetric (resp. Novikov) algebra structure on $E$
such that $A$ is a subalgebra of $E$ if and only if $E$ is isomorphic to a
unified product of $A$ and $V$. Two cohomological type objects
$\mathcal{H}_A^2(V,A)$ and $\mathcal{H}^2(V,A)$ are constructed to give a
theoretical answer to the extending structures problem. Furthermore, given an
extension $A\subset E$ of left-symmetric (resp. Novikov) algebras, another
cohomological type object is constructed to classify all complements of $A$ in
$E$. Several special examples are provided in details.
