Building a house without equity or how does one proceed?

One can do a lot, as long as the bank is convinced that it will get its money back.

Either one has alternative collateral, a very good income, or a correspondingly high repayment. Or a combination of all three.

Can't these additional costs also be financed?

You can (there are people who finance phones or refrigerators), but the number of providers who do this is very small. You only get 100% financing with top creditworthiness, meaning an initial repayment rate of 3% p.a. Depending on the federal state and circumstances, you need more than "110%", which then only includes the "hard" additional costs, but no fees, no furniture, etc. In the end, you want to have 120% financed. Calculate what that means and whether you can afford the rate (currently, I would assume an annuity of 6% of the loan amount). Note: it is not you who decides whether you can afford the rate, but the bank at its own discretion.

not to forget that a 100% or 120% financing also results in significantly higher interest rates. if you also choose a long fixed interest period, you basically no longer benefit from the low interest rate over