Own contribution, how much to apply?

Is a question of your goal:
If you want to calculate the equity contribution for the bank loan Bank - go with a large number.
If you want to calculate the equity contribution for your budget security - go with a small number.

even if it was a while ago....

with the own contribution = like equity, you can get into a "better risk category" at the bank = better conditions.

many banks have this "magic limit" of own contribution of 15,000 EUR, but that only means that you don't have to "prove" more than that.

if you have a statement from the architect/house building company showing that the own contribution is higher, many banks are allowed to consider it.

I have already calculated a case at ING with 85,000 EUR own contribution, it was no problem because there was a nice statement from the architect, and half the family works in the skilled trades sector (so also plausible).

effectively you are building almost turnkey. Most people do the painting and such themselves anyway, and the outdoor facilities are added to the ancillary costs and not to the house price. Just look at what kind of stones you want, calculate the material cost, and add a little extra on top.