The jpoc guide to fixing your car in Germany.
Well, you are allowed to work on your own car but only in the privacy of your own driveway or garage. In particular you are not allowed to do this sort of thing at the side of the road. If you do, you will get into trouble very quickly. Not just from a randomly passing police car but it is quite likely that your neighbours will complain to you or directly to the police.
Working on a private drive or inside your own garage of course gives you a lot more leeway but don't be surprised if a nosy neighbour wants to check that you are not allowing used oil to spill onto the ground or into the drains.
If you are renting your accommodation, you will probably face further restrictions. It is very common to see a restriction to the effect that, while you have a reserved parking place, or even a spot in the communal underground garage, you are only allowed to park a car there and you may not carry out work on the car.
How much flexibility is there in this?
Well, suppose that you have a reserved parking place on some hard-standing outside your building or in the underground garage. I bet that nobody would complain if you pumped up your tyres or swapped the summer tyres for the winter ones. But they might! On a hot day, the metal frame of a car foot pump can make an appreciable hole in a tarmac surface. German landlords worry about things like that. Also, if you do it on a Sunday afternoon, you might upset somebody who thinks that it is a day of rest. I'd not recommend that you try topping up with oil in your underground garage as that is much more likely to cause somebody to complain and I do know of somebody who got into trouble for changing over from summer to winter tyres while parked on the street.