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Let's say I have a car and I want to make some modifications myself without a professional. It can be just for fun or personal research/testing. I may just add a train horn, an auto-breaking system, a semi-self-driving system, something like that. Assuming I don't make a change that clearly and directly makes driving dangerous like disconnecting the breaks.

Is it illegal anywhere in Europe to modify one's own vehicle (car, motorbike, etc) like that?

  • Do you intend to operate the car on a public road? Do you expect to ever sell or give it to anyone else? – Nate Eldredge Jan 3 at 9:13
  • The vehicle is intended for use on in public but not later sold. – John Zhau Jan 3 at 9:51
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It will very much depend on what you modify. If you only drive on your own land, that's mostly fine (possibly not fine if your own land is a road accessible to the public).

What will cause you problems:

The manufacturer has a general permission to distribute car models with certain characteristics in your country. There's trouble if your modification means your modified car doesn't have these characteristics. Your car insurance is for a certain car model.

Your insurance may be invalid if you modify your car. As "boy racers" were mentioned, part of the insurance cost is based on the type of drivers this car model attracts. If you change it from "boring old family car driven by boring old dads who never have accidents" to "exciting boy racer car driven by maniacs who crash their car regularly", you would be expected to tell your insurance so they can adapt their insurance.

As far as passing your MOT test goes: If you don't advise the tester of the change, and therefore the fact that your car is unsafe is missed, your MOT is not valid. Same in the time between making changes and the next MOT if your changes made the car unsafe. Usually that will be found out at the worst possible time: After you are involved in a costly accident.

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To my knowledge, any car modfication that is not simply using manufacturers additions as intended (for example, removing a car seat when it is intended to be removed) is very much illegal until you report the change to your state's vehicle registration office and add the added part's manufacturer's documentation. Even things like new LED headlights (upgrade from old non-led) need to be approved by them. Also car color, window tints, etc. And it's not always easy to get their approval. So my bet would be that yes, it is illegal to do any modifications pretty much anywhere and is quite strictly enforced.

EDIT: As per @NateEldredge comment above, these only apply if you intend to use the car on public roads.

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    I’m not sure about anywhere else, but in the UK so long as the vehicle passes the MOT test (annual safety test) and is insured (you have to declare modifications) then it’s legal to basically go wild on your car - there is a huge after parts market for the “boy racer” scene, including fitting loads of things a manufacturer never intended to be fitted. Very little of this needs to be reported to the DVLA - engine swaps for a non-identical engine perhaps, but that’s more to do with taxation than preventing modifications. – Moo Jan 3 at 9:56
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    Well, that's very much not true for Germany and Slovenia. In Slovenia, any change that affects car's appearance needs to be certified. That includes changing car documentation with the appropriate authority. UK appears to be different in that regard, let's see if any other countries have laws like that. – user29225 Jan 3 at 10:31

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