If its a kit built microlight, and operated on a PFA administered permit to fly, then the PFA stipulate a maximum empty weight as well. IIRC this is based on an 80kg pilot (and passenger if a 2 seater) and one hours fuel.
For the Icarus this works out at 265kg or 268kg depending on which Rotax engine (80hp or 100hp) is fitted. This is in section 15 of
the PFA Data Sheet for the C42.
I'm not sure about factory built microlights as the BMAA look after the permit to fly for these.
Brooklands