You could ask the builders nicely, they may let you in. Wear hi viz and follow a vehicle. Go back at night and climb over the fence. Leave it for now and return when the site is completed. Act daft and just run in. These and other methods helped me finish Sheffield.
I mean… IS it private? I am not up on my Norway all that much, but I know in the states you can usually hop on a county GIS and confirm ownership. I work in the construction industry, and I could absolutely see a world where a dead end/cul-de-sac is temporarily closed for road work/private development if there are no other properties needing access.
It may be just as you say, but I’m always a little dodgy about editing OSM unless I can confirm something like that.