I bought a car from a friend a few months ago and we just made a written document ourselves, without going to a notary or anything. Now I want to know if that's legally valid to change the vehicle's papers or if I need to do something else.
It depends a lot on the country or province you're in, where are you from? Because in some places a private document signed by both of you is enough to do the transfer at the motor vehicle registry, but in others they're gonna ask you to do it in front of a notary or that you have certain specific requirements. The safest thing is for you to go to the traffic authority or vehicle registry in your area and ask them what exactly they need to change the papers to your name, because if you do it without meeting the requirements they ask for, later you could run into problems when you try to resell the car or in case of an accident.
The truth is what you wrote is already kind of answered above, but add to that even though a private document works for the transfer in some places, it's better you do it with a notary anyway - you avoid headaches later if something goes wrong or your friend changes their mind, and the investment isn't that much.
I've been through the same thing, and the reality is that a private document signed by both of you works in most autonomous communities for vehicle transfer, but here's what nobody will tell you: before going to the registry, call the Traffic Authority in your province directly and ask what they specifically need, because each one has its quirks and that way you avoid surprises. What is advisable is that this private document is well written out (complete data, signatures from both, clear date) and that you have witnesses or at least do it in writing so there's a record, even though in 2026 many registries accept the bare minimum if it's actually signed by both parties. A notarized deed gives more legal security, but if you have the paper done properly, it should go smoothly for you.
I went through something similar a couple of years ago and honestly I'd recommend checking with your province's motor vehicle registry specifically, because the requirements vary quite a bit from place to place. What most people don't mention is that even though some registries accept a private document signed by both parties, if there's a problem or dispute later (like your friend says it was a loan and not a sale), a paper without a notary doesn't hold up well legally. My sarcastic but honest advice: spend a bit more now on a public deed or at least on a signature certified by a notary, because when you actually need that protection, the 50-100 bucks you saved end up looking ridiculous.
Basically, yeah, the private document does work, but there's an important nuance that several people are overlooking. The paper you two drew up is legally valid, but that doesn't mean it's enough for everything. When you go to the traffic registry to change the vehicle's paperwork, they're going to check that it's properly signed, that it has the correct information from both of you, the license plate, the chassis number... all that. If you did it carefully and everything's clear, they'll probably accept it without any issues. Now, the thing is that if something goes wrong later (a complaint, a financing problem, whatever), a private document between two mates is way weaker than one notarized. I'm not saying anything dodgy happened, but legally speaking, without a notary there's less protection for either of you.
My realistic advice is that you check directly with the General Traffic Directorate in your province or your local vehicle registry, because even though most places accept the private document, some ask for more specific stuff. Some registries tell you to just bring the paper as is and you're done, others ask you to have it certified, others ask for a specific contract template. It's half an hour on the phone and you'll be sorted. If for whatever reason the registry says it's not good enough for them, then you'll have to go to a notary, but I'm pretty sure they'll accept it. The important thing is that the document is done properly: full names, ID numbers, correct license plate and chassis number, the amount of money, date, and both signatures clear.
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