Financing single-family house new construction your assessment as of today!

Let's get back to the essentials: 1600€ installment is not possible (interest and unplanned buffer), the original poster has excluded a higher installment.

So the project is dead anyway, or am I misunderstanding?

"Employee discounts are also subject to a tax exemption amount of 1,080 euros. If the final amount is higher than the exemption despite the employee discount, it is considered a taxable monetary benefit.".

I wouldn’t make a big deal out of it. In any case, it is not entirely legal. You would actually have to declare the profit as income and pay taxes on it.

I rather wonder which craftsman would go along with that. Mine definitely did not want to install "foreign" materials.

I also work in the building materials trade. Our calculation is also equity + 3%. Of course, plus VAT.
The note about the exemption limit is correct and important; audits or payroll tax audits examine employee accounts closely. However, it is the employee’s responsibility to pay attention to this, not the employer’s. Taxation of any monetary benefit is the employee’s tax obligation.

Regarding the question of providing materials, I can only say that I do the same. But only for the significant large items. Small materials are brought by the craftsmen themselves, and the heating engineer also gets his supplies from the wholesale trade.
There has never been a problem with warranties so far; if problems have arisen, we have always been able to resolve them reasonably together.

Let's get back to the essentials: a 1600€ installment won't work (interest as well as an unplanned buffer), the thread starter has excluded a higher installment.

So the project is basically dead anyway, or am I misunderstanding it?

If you base the answer to this question purely on the ratio of income to loan, I would actually still consider it doable - if one accepts the previous calculation.
BUT
I am still missing some information/details/priorities specifically regarding this so that it makes sense:
- The incidental purchase costs for the land (120k) are probably around 10k if not bought within the family - but this is nowhere mentioned. Are these costs already included in the land price of 120k?
- I have not been able to identify any ancillary construction costs so far either
- As mentioned several times, there also appears to be no further buffer.
- If the desire for a home has existed for a longer time, I wouldn't have bought a vehicle in a price range that already requires financing. (Last year I swapped my dream car for a cheaper one to generate liquid funds)

With the equity, the land is paid off, so 408k remain for the entire house including ancillary costs - I consider this unrealistic despite material purchases from the employer and contracting companies. Assuming 70k ancillary costs and 10k incidental purchase costs for the land, 328k remain for the house, i.e., about 2,340€/m². There should only be a few places left in Germany where building can be done at this price per square meter. Our project is already 1,100€ above that and we don’t even have a garage.

If the tax office then also collects the GW advantage, the whole thing will be a disaster.

My advice at this point based on the information available so far:
Buy the land now with your equity.
Put as much money as possible aside to save up equity again. Reduce your costs to a minimum. Possibly even consider side jobs (we also did this – just don’t overwhelm yourselves).
Sit out the current situation and plan your project including the building permit in the meantime, then you’ll be ready to start when the right time comes and you’ll gain a lot of insights during this time which you probably don’t have today.
You are young, so in my opinion, starting your family planning in 2-3 years is absolutely okay.

If construction costs then exceed your financial scope in the future, you can sell the land and switch to an existing property.

I will clarify the monetary benefit accordingly to avoid any nasty surprises, I am also allowed to put the invoices in my wife’s name, maybe then it won’t count as a monetary benefit, but I will discuss that with the tax advisor, thanks for the tip! A friend of mine in my area has about 2900 sq m with a basement, we are building without a basement with a discount, hence the 2500 assumed! Whether that will work we will see after we have been to the architect (also an acquaintance of ours)! Regarding the buffer, money was offered to us from the family side, but I would like to avoid it if possible! We could also plan with 120 sq m, that would be enough for us, so I also see that as a buffer!

"Employee discounts are also subject to a tax exemption amount of 1,080 euros. If the final amount is higher than the exemption amount despite the employee discount, it is considered a taxable monetary benefit."

I wouldn't make a big deal out of it. In any case, it is not entirely legal. You would actually have to tax the profit as income.

I rather wonder which craftsman would go along with that. Mine certainly did not want to install "foreign" material.

I will clarify the monetary benefit accordingly to avoid any unpleasant surprises; I am also allowed to put the invoices in my wife's name, maybe then it won't count as a monetary benefit, but I will clarify that with the tax advisor, thanks for the tip! A friend of mine gets about 2900 per sqm including basement in my area; we are building without a basement with a discount, so 2500 was assumed! Whether that works out, we will see when we have been to the architect (also an acquaintance of ours)! Regarding a buffer, we were offered money from the family side, but if possible, I would like to refrain from that! We could also plan with 120 sqm, that would be enough for us as well, so I also see it as a buffer!

It is also considered a monetary benefit if discounted building materials are given to the wife, husband, or other relatives. Therefore, there is little room for maneuver. The question is always whether someone will catch it at the next audit. If you purchase goods worth 150,000 euros, that will definitely be an issue somewhere. If it is just a few thousand euros per year, then probably not.
If you process the sales through a miscellaneous account, you have to note that the same delivery address will always appear. That can also attract attention... There are definitely auditors who get creative when clarifying facts. Sometimes they ask around incognito in the warehouse or with the information staff; they already have some very good ideas.
Overall, I wouldn't get too worked up about the topic.