interesting topic
I see several problem areas:
We both save about half of our salary every month.
I have had this salary for one year and my boyfriend for five years (increasing each year).
So €30,000 saved together last year and your boyfriend about €12,500 per year before that, over 4 years = €50,000, total equity for both of you = €80,000. Do you have this money, yes or no? If no: first problem: you are lying to yourselves about the savings rate. Down payments for the cars, etc... you know how it is. The problem is, the bank also calculates such things afterwards.
I don't want to fully disclose all our savings deposits, but as I said, we add €25,000 for now and keep a buffer each for emergencies. (You never know what else could come)
Very important: separate construction buffer and private buffer. How high are the buffers? The bank might want to see if you are realistic!
With a prefabricated house company, we “have to” more or less let them build, since both of us are not home from 6 in the morning until 8 in the evening, as our commute is quite long and we therefore cannot take care of much.
We are now trying to save on the bathroom, the flooring, and the painting work and are awarding these tasks to acquaintances who work in these areas.
Problem no. 2:
What does the construction service description say? The missing painting work may still include fine interior plaster, filling work, connection joints, fabric transitions, etc., and not just the paint coat, unfortunately many confuse this. Especially with raw ceilings, it gets really nasty and is hardly manageable with self-labor. Check the construction service description for what it says about painting work, especially ceilings.
Flooring is also no fun. Having living room tiles laid with comparatively normal 120x25 tiles in wood look cost us about €5,000 in the budget. Just as an example.
The cars have a guaranteed buyback right, so we return them after a few years and do not pay them off completely.
Problem no. 3: you wrote above: “longer commute” and in a few years you will be without cars. And then? Buy with a final installment? Money planned? Or down payment for new cars planned? Then the installments still continue anyway and you basically keep paying off. That means the idea mentioned earlier to list the cars as debts was correct. The fact is, you are currently and also in the future burning a lot of money on your mobility.
4th problem: again about the equity, sorry! Besides the buffers and the completion self-labor, for which you basically have no time and hardly any money (relatives also need to buy materials)
you want to pay everything else from this:
We use equity for incidental costs, like notary, property transfer tax, etc., as well as construction incidental costs.
We want to keep a little bit free for furnishing and outdoor facilities.
You also mentioned a surveyor somewhere... and not to forget:
The equity we want to bring in is about €25,000, the rest remains as said on our accounts for furnishing kitchen, etc.
The kitchen too? WTF? In my opinion, this doesn't add up at all. As others have already said: you are not giving us all the numbers, so it is a guessing game. But assuming you have a total of €45,000 for everything mentioned above (I can literally feel the number) including construction buffer and private buffer (the washing machine... you know), that somehow does not fit.
PS, if the credit decision in your case (for whatever reason) was made as emotionally as my analysis and less based on cold facts, I am unfortunately not surprised by the rejection.