based on the size of the unit you have you should get small business rate relief, also based on the size of the unit and its rateable value you might not pay rates at all. Speak to your rates office, ring local council and they will give you the number, I wouldnt tell them its for personal use as business premises can not just be changed to personal use etc with out forms and visits.
A few words of advice when renting a unit for motor trade use as this is how they will see it. You will need public liabilty insurance, also make sure you get a very very secure unit as the local scum love breaking into workshops for tools cutting off CATS etc, we have spent a small fortune on alarms CCTV etc just some friendly advice