  It is known that if the underlying iterated function system satisfies the
open set condition, then the upper box dimension of an inhomogeneous
self-similar set is the maximum of the upper box dimensions of the homogeneous
counterpart and the condensation set. First, we prove that this `expected
formula' does not hold in general if there are overlaps in the construction. We
demonstrate this via two different types of counterexample: the first is a
family of overlapping inhomogeneous self-similar sets based upon Bernoulli
convolutions; and the second applies in higher dimensions and makes use of a
spectral gap property that holds for certain subgroups of $SO(d)$ for $d\geq
3$.
  We also obtain new upper bounds for the upper box dimension of an
inhomogeneous self-similar set which hold in general. Moreover, our
counterexamples demonstrate that these bounds are optimal. In the final section
we show that if the \emph{weak separation property} is satisfied, ie. the
overlaps are controllable, then the `expected formula' does hold.
