Marital agreement on inherited assets

[betelgeuse]

Having a signature in a paper (obviously doubly witnessed etc), would that be enough?

Making a deal that violates a law is not valid. (think tax evasion and so on). Therefore the Maistraatti needs to be involved.

It also plays into preventing fraud. There's enough messy divorces that I think otherwise you would semi regularly have the case that the other party has forged the signature and paid off a couple witnesses. You can't register a prenup once a divorce has been initiated.

[Rip]

No. The link already posted by CH shows that you must register it at any Maistraatti.

http://www.maistraatti.fi/avioehtosopimus

+ note that maistraatti makes no legal check of the validity of the contents of the paper (= could be later successfully contested in a court)