Is a construction financing advisor present?

You would need to please write more precisely what is unclear to you.

Does the muscle mortgage only increase the equity or the construction costs by the same amount?
If it increases the construction costs, do I then have access to this amount for material and labor costs?

The muscle mortgage increases the construction costs and on the other hand the equity. (Also kein Fremdkapital und schon gar nicht in Form von Geld)

In other words: labor... and specifically yours... without compensation.

I would assume that your acquaintance aimed to push the loan-to-value ratio down to 80% (which results in better interest rates). With 15k own contribution and 46k liquid assets, you have 61k equity versus 308k debt (loan). That is almost exactly 20% equity, or 80% loan-to-value ratio. If you put in less yourselves, you won’t reach this "magic limit."

Theoretically, you could also contribute less cash and more own work (labor) as long as it sums up to 61,000 or the 20:80 ratio in the end. But that’s just for better understanding!

Your "muscle mortgage" must therefore be accounted for on both sides. Once as contributed means (at the bottom of the statement) and at the same time as the "value" then inside the house (at the top of the table). Instead of your labor, you could (hypothetically speaking) also contribute a pile of roof tiles as "own work" if you happened to have them lying around, or a solar system which you won in a prize draw – it’s only about a "value" going into the house that is not paid for by the bank’s loan.

You only have "access" to the 308k loan and limited access to your contributed cash, whereby the bank in your case will probably insist that you primarily spend your own cash before the loan is used. In other words: The money is gone as soon as the house belongs to you. 313,900 + 5,876 + 19,097 = 338,873 - 45,873 = 293,000. So the bank initially keeps 15,000 that have not been drawn yet.

What I think is now confusing you is the term "own work." Your acquaintance represented this above as a total item because it would have been difficult (and confusing) to divide it in advance among the individual areas. Otherwise, it would have said something like:

- Painting: 5,000 (materials) + (5,000 (labor costs) - 5,000 (own work)) = 5,000 euros
- Flooring: 3,000 (materials) + (7,000 (labor costs) - 5,000 (own work)) = 5,000 euros
- Outdoor area: 2,000 (materials) + (8,000 (labor costs) - 5,000 (own work)) = 5,000 euros

---> Total additional loan requirement for "Painting/Flooring/Outdoor area" altogether 15,000 euros

Or in other words: Instead of "own work" and "outdoor area," simply "materials" could also be listed above.

When you then buy laminate or boxwoods at the hardware store, you submit the invoice to the bank and they pay you the amount from the remaining 15,000 loan until it is completely used up.

You would have to please write more precisely what is unclear to you.

Does the muscle mortgage only increase the equity or the construction costs by the same amount?
If it increases the construction costs, do I then have access to this amount for material and labor costs?

I do not understand the entire text, and I also do not see a specific question about the text that is not understood.

He wants to spend 314,000 for the house plus additional costs of 25,000, so 339,000 in total, and use 35,000 equity, therefore borrowing 304,000 and later calmly choose floors, paint, and kitchen from the clearance market.

Now his acquaintance comes along and adds 2x 15,000 to the costs, "steals" 10,000 cash from him, talks about personal contribution and thereby also increases the required loan by 4,000 euros.

Now he wants to know why the acquaintance is doing this (if I have understood that correctly).

The house costs 368 T€, it cannot be calculated cheaper and it does not matter where the costs come from. The bank does not finance a half-finished house. These T€ 368 must be financed. This can be done with equity/debt as well as EL – when what is used must be agreed with the bank.

EL are only labor costs, material also belongs with that.