Let me be clear about one thing up front: this is not legal advice, but how I myself think about the financial sides of moving in together or marrying. It is an area where the misconceptions are many and expensive, because almost nobody reads up before love moves into the same flat. And the most common misconception is that cohabitants (sambos) have roughly the same protection as married couples. They do not, at all.
The Cohabitation Act (sambolagen) only splits the home and household goods you acquired to live in together, and it splits them equally regardless of who paid. Everything else, savings, the car, what you owned before, is yours. Sambos do not inherit each other at all without a will. Marry and the main rule instead becomes that everything is split equally, and you get automatic inheritance protection, with children from a previous relationship as the key exception. Either way: write it down when you put in unequal down payments.
I walk through what sambolagen actually splits, why sambos do not inherit each other, what marriage changes, a worked example of unequal down payments, and the paperwork that protects you. If you are buying a home together, run the numbers in the mortgage calculator.
The Cohabitation Act splits less than you think
Sambolagen covers just one thing: the home and household goods you acquired to use together. If you split up and one of you requests a division, that property is split equally, regardless of who paid for it. Everything else falls outside: your savings, your assets, your car, what you owned before you became sambos, and a home one of you bought that was not meant for your shared everyday life.
That "regardless of who paid" is exactly where it hurts. Buy a home together to live in, and one of you put in a much larger down payment, and the value is still split equally at a separation, unless you have written a cohabitation agreement (samboavtal) that says otherwise. It is not a theoretical problem, it is the most common financial dispute I hear about between sambos.
Sambos do not inherit each other
This is the most dangerous gap. Sambos do not inherit each other at all. If one of you dies, everything goes to that person's heirs, first of all the children, not to the surviving sambo. The only way to give your sambo anything is a will, and even then direct heirs (bröstarvingar) have a right to their statutory share (laglott), meaning half of what they would otherwise get. If the deceased has children from before (särkullbarn), they also have the right to take their inheritance immediately.
What marriage changes
Marry and the main rule turns around. Everything you own becomes marital property (giftorättsgods), and at divorce or death the combined net is split equally, not just the home. If you want to keep something outside, such as a business or an inheritance, you write a prenuptial agreement (äktenskapsförord) that makes that property separate (enskild). Marriage also gives automatic inheritance protection: spouses inherit each other before common children.
The exception is the same as for sambos: särkullbarn. A child that only one spouse has gets the right to take their inheritance immediately on the parent's death, before the surviving spouse, unless they waive it in the spouse's favour. It is precisely särkullbarn that are the reason even married couples sometimes need a will on top of the law's protection.
Worked example: unequal down payments
Say you buy a home together to live in. One of you puts in 400,000 kr as a down payment, the other 100,000 kr. On paper you own it half each. If it ends and the home is divided as cohabitation property, the surplus value is split equally, so the one who put in 400,000 kr gets back just as much as the one who put in 100,000 kr. The 300,000 kr difference has, in practice, become a gift.
The solution is simple and cheap: write a promissory note (skuldebrev) documenting the larger contribution, so it is repaid before the rest is split, and ideally a cohabitation agreement covering the home. Then each of you gets back what you actually put in, and only the shared increase in value is split equally. It takes an evening to sort out and can save you a truly bitter conflict.
The paperwork that protects you
Four documents solve most of it. A cohabitation agreement if you want to place the home and household goods outside sambolagen's equal split. A promissory note when you put in unequal amounts, so the difference is a debt and not a gift. A will if you want a sambo, or a spouse with särkullbarn in the picture, to get more than the law gives. And a review of the beneficiaries on pensions and insurances, since they are paid out beside the inheritance to whoever you have named.
My take
Here is how I think about it: sambo or married is a personal choice, but the protection does not automatically follow the feelings. If you are sambos with a shared home and unequal contributions, treat a cohabitation agreement and a promissory note as just as obvious as home insurance. If either of you has children from before, talk about a will whether you are sambos or married. And remember that the cheapest time to sort this out is now, when you agree, not when you do not. This is, as I said, not legal advice; for large amounts, let a lawyer draft the documents.