  The last decade witnessed an increasing interest in axions and axion-like
particles with many theoretical works published and many new experimental
proposals that started a real race towards their discovery. This paper is the
Conceptual Design Report of the KLASH (KLoe magnet for Axion SearcH) experiment
at the Laboratori Nazionali di Frascati (LNF). The idea of this experiment has
been stimulated by the availability of the large volume superconducting magnet,
with a moderate magnetic field of 0.6 T, used in the KLOE detector at the DAFNE
collider. The main conclusion we draw from this report is the possibility to
build and put in operation at LNF in 2-3 years a large haloscope with the
sensitivity to KSVZ axions in the low mass range between 0.2 and 1 $\mu$eV,
complementary to that of other experiments. Timeline and cost are competitive
with respect to other proposals in the same mass region thanks to the
availability of most of the infrastructure, in particular the superconducting
magnet and the cryogenics plant.
