Construction costs approximately 500 TE minus 30 EL and 40 equity capital means 430 TE to be financed. Officially, you only want to show the bank your salary, 2350 euros. No bank will finance that, sorry to say. Nada!
And even with 3800 euros combined, it will be very hard to find a bank. The only chance though! Personally, I would find the whole thing too tight.
That unfortunately won’t fly. You have to plan for the kitchen, garden, upgrades, and buffer. We just had that in another thread, what else can happen in the worst case.
So my calculation is correct.