Is the financing plan realistic?

Presumably - but you are wrong. Such constellations/relationships are still sufficiently financed.

Where do you get the information from? Do you know many people who have been financed exactly like that now?

I work in the field.

I also know some [da].

I personally fully financed my apartment including additional costs in 2012 (117% financing). However, the amount was so small that I would only have received a smaller sum with a surcharge for a "small loan." Since I renovated immediately, everything worked out well.

The same applies to the land, which I financed a year ago: fully financed including additional costs (112% financing), but here too it was only about a fraction of, for example, the mentioned 300,000.

Currently, the buffer concerns me because it would work without it with 20% equity and 80% loan-to-value, but with the buffer it leads to full financing, which means a 0.25% higher interest rate.

Regards

He is presumably missing significantly more than 30,000 euros of his own funds to reach a financeable loan-to-value ratio (assuming the total costs amount to 430,000 euros without buffer).

I see it that way too. For our reserve, we also just "reduced" equity and gave up 0.1% better interest rate. 70% instead of 60% loan-to-value. Makes me sleep more peacefully.. well rather "less restless".

Regards, Steffen

Everyone just has a different need for security - you're more on the overcautious side anyway.