Here in Germany it's okay to do your own modifications but you'll need papers with the parts you're using saying that they are TUV approved. The vehicle will have to be inspected to see if they are fitted properly.
You can get away with fitting stuff and not tell 'em but it will fail on it's next inspection. Besides the fact that it makes your insurance void.
It can be a headache at times but you won't have trailing arms on your motor that can snap off ...