even if everyone here is right, the tone is just not nice. he really messed up and everyone is just trampling on him even more. as he told it, it will never work out of course. even the stingiest Hartz IV home builders don't get through without extra costs during construction. then complete items are missing and even if you put up the wallpaper yourself, the material will easily cost 2000€ or more (that's quite a few sqm to cover...). and the flooring was probably also withheld and the outdoor area costs a lot of money, even if you do everything yourself.
just as a reference for us:
house with the general contractor 200,000€
land 63,000€
house costs on the handover day: 340,000€. but we also went all out, outside there wasn’t even a doorstep included in the price. the outdoor area now roughly costs 30,000-35,000€ finished including carport
we planned with 330,000€ including a 15,000€ reserve. the earthworks (and digging for sewage) cost a ton of money. also, we didn’t calculate 1-2 inspections beforehand. I wanted fingerprint and an overlapping-leaf front door, that just costs 4000€.
in any case, good luck. otherwise ask your lender if you can get additional financing. however, all other costs will come again as they also have to be recorded...
one thing amazes me quite heavily now: why on earth did the bank sell it to you like that? they require a calculation and know that this is nonsense...