I don't think anybody who got tig,mig welder or arch welder in there household are declared them to anybody....
Many of them get discovered when neighbours complain to the electricity company about dipping lights. In these cases the DNO has every right to disconnect the supply if the offender continues to use the equipment after being given notice to cease using it.
it's NOT COMERCIAL GARAGE
Private or commercial doesn't matter, if you affect the voltage on the local network by using un-declared equipment ( motors and / or welders ) then you are at fault.