Woop Woop, anyone who vacations on Fehmarn is already instantly likeable to me! I also like to hang out in Burg and stand on one of the few remaining cutters at half past five in the morning, nothing better ;-) Well, a bit off-topic so far.
Regarding your concern: I am currently in a "similar" situation as you. Wife, one child, a few hundred thousand euros in loans yes/no - that can keep you awake at night.
Anyway, we are also buying a plot of land 150€ below the standardized land value, so I want to address your question whether banks consider the difference as equity. That has occupied me and the answer is yes and no...
My ultra-conservative house bank (the red faction) for example does not consider it equity. They calculate based on a financing ratio, i.e. capital demand from construction costs and assign you the corresponding interest rate according to the current terms sheet. If you’ve already dug a bit through the forum here, you have probably come across the term construction costs. Basically, first the plot at purchase price (including outdoor facilities, garage, carport, ... and the house (or everything that does not fall off if you turn the house upside down --> furniture and kitchen, for example, are not counted). But the bank still of course sees the difference at the latest during collateralization. So either way a great thing with positive effects if you win the bid! Then there’s usually no additional land registry entry at the parents or something like that needed so quickly.
I also once went to one of the big brokers about the topic. We acted as if the plot was equity at the standardized land value and I was also confirmed that some banks also apply this in their calculation of the loan-to-value ratio and not just during collateralization. These seemed often to be the "big" financiers who still throw liquidity around and rather less the local house banks.
Seen that way, both are possible.
At least currently you wouldn’t have any trouble finding someone to give you the money. Whether you feel comfortable with that is another question.
For me right now it is truly an emotional rollercoaster. Today euphoric and hot as hot oil, that it’s finally starting, and tomorrow again full of doubts (mostly completely irrational) whether it will all somehow work out.
By the way, we will finance about 530-550k€ on 5,000€ income. Sure, income will rise too, that’s why you bend over every day, and all that without profit sharing and holiday/Christmas bonuses and other special payments, but with half a million you do get a little dizzy for a moment...
Cheers!