New construction with existing parental house - how to finance?

First of all, thanks so far. Yes, my father-in-law actually has 3 master craftsman certificates.

We now have an appointment next week with a financing advisor, where we should roughly outline the costs.

We already have a rough idea of the costs for the plot + interior fittings, but we are missing a ballpark figure for the construction itself.
About the project:
It should be about 200 sqm (3 children’s rooms), 1.5 stories, high knee wall (min. 1.20m), double garage 50 sqm. Otherwise underfloor heating, no smart home, preferably solar power on the roof, relatively many windows (including some floor-to-ceiling ones).

Is it even roughly possible to estimate this, considering the extensive own contribution?
My father-in-law would need a drawing to calculate, but of course we are not that far yet, we first want to clarify whether it is financially even possible.
Thanks so far

can you even roughly estimate that, considering the many own contributions?

Yes, if you can estimate the material and the rest of the family tells you their prices.
That could also be the architect who is handling your building application.
Otherwise, the father-in-law should know as well.

I think I will sit down with him on short notice and roughly calculate everything. Since we ourselves naturally have no construction experience, what is the normal process from the idea of building to the start of construction?

If the Sparkasse is not cooperative/interested, then it will just be another financier.
As a civil servant with a debt-free single-family house as equity, that shouldn't really be a problem....

35k€ for a plot of land including all additional costs? I'm envious. Here in the heart of NRW, you with a lot of luck can build a garden shed on it... or it’s in the middle of nowhere.
But I am a bit confused regarding the other information...

The main work will be done by my father-in-law (master mason, master tiler, architect, structural engineer, etc.), who has already built several houses in his life and of course wants to take on this project with us.

Since we obviously have no construction experience, what is the normal process from the idea of building until construction start?

Can it be roughly estimated considering the many DIY efforts?
My father-in-law would need a drawing to calculate it, but we are of course not that far yet and first want to clarify if it is even financially possible.

Unfortunately, we do not have such expertise on hand, but we have already developed a somewhat more concrete idea ourselves. Your thread, on the other hand, is a month old and seems to be becoming more diffuse rather than clearer?
You have a plot in mind, wouldn’t you have already discussed most of the details within the family and sketched them a hundred times?
That somehow doesn’t add up.

And regarding the long-term working hours account: How much are you allowed to accumulate there, is there no cut-off?
Even if you have 200 hours saved, those are used up in about 5 working weeks (calculated at 40h/week)...
Or can (do you plan to) go significantly into the negative over 1-2 years of construction time?