What do you mean by "not marrying like THAT"? As far as I know (and it was exactly the same for us), both say "Yes, I do" and the deal is done. Whether with house construction or without...
I see it like Kaho – a shared house binds more than a marriage. Without a house and shared purchases, a divorce is no big deal – a separation with a house, whether married or not, is likely far more complicated. So, in short: I should consider BEFORE starting the house construction whether I seriously want to grow old with the person. If in doubt, I would either build alone or, if that's not possible alone, stay away from it. And if I can confidently say "Yes" here, I can also marry and enjoy all the advantages that come with it. You still have to arrange plenty, but some things are taken care of by this simple "yes" (especially the matter of death is clear and resolved in favor of the partner).
We didn't need a marriage, but it doesn't hurt either, it didn't change our relationship and the party was cool