best to explain to your sparky exactly what you are going to be doing. if you are legal, there is no harm explaining. if you are not legal, tell him the power is for metal working tools, such as mills, lathes, chop saws, welders, etc, which draw a fuckton of juice. you want to take into account pumps, fans and anything else that uses juice in your grow room. add it all up, add 30%, and that is what your wiring wants to handle minimum.
as for the actual install, the wiring may already be able to handle the load, it might just be your RCD that needs the upgrade. if additional wiring needs to be done, it depends entirely on your layout. you may have wiring ducting in place which is a simple thread through, or he may have to chase through blockwork or whatever. he will explain it to you, however, always have a certified professional do it and get a full account of what work has been done, so if your shit burns down, you have grounds to sue.