Process of property purchase/financing

at the time of signing the contract with the financing bank.

Then I will phrase the question differently: when is the earliest one can sign the contract with the financing bank?

Then I will phrase the question differently: when can you at the earliest sign the contract with the financing bank?

Always. It's just your risk to lock in the conditions too early or to get others too late. The calculated conditions are - I believe - only valid for about 2 weeks. After that, there may be new conditions regarding the interest rate. If you lock in too early, i.e. sign, the availability period may possibly not be sufficient, so you will have to pay availability interest, which is not uncommon if the construction takes a little longer. I would budget 1,000-2,000 € for that.

Always. But it is your risk to lock in the conditions too early or to get others too late. The calculated conditions apply – I believe – only for about 2 weeks. After that, there may be new ones regarding the interest rate. If you lock in too early, that is, sign, the provision period may not be sufficient, so you will have to pay provision interest, which is also not uncommon if the construction takes a bit longer. I would budget 1,000-2,000 € for that.

So, can I already sign the contract with the bank with a construction cost plan in hand? And only then take care of the building permit and so on.

When you submit the construction cost plan, later payslips, etc., it will first be checked whether it makes sense. The contracting party wants to finance something real and not something half-finished. The banks, for example, also want to have the planning documents of the [BUs]... Ultimately, the information and steps interlock. And it also counts: you have to adhere to the signed contract, so you must be sure that the construction will correspond to the approximate construction costs. Your bank should provide you with all of this in writing, basically a list :)

When signing a contract for a turnkey company for, for example, a house for 300,000, one should only do so if one is sure to obtain a correspondingly high loan.

The dilemma here is unfortunately that with the usual KFW loan in tow, a completed "kfw requirements" form from the construction company must first be available before the bank gives a final financing commitment. However, these kfw requirements can only be available once it is clear what the house looks like and someone has calculated all the stuff. Consequently, costs arise before you know whether the bank will even pay for the fun. There are also a few other things like enclosed space and such jokes, which you cannot know before the floor plan is finished. Especially the big online brokers like [e.g. Interhyp] really want a lot of paperwork and also have a tendency to exaggerate (for example, they wanted proof of WHERE the equity comes from - none of their business, I submitted a bank statement from the overnight money account, that should be enough). In the end, it often goes something like this: - chosen and reserved land - talks with the banks where you want to get an offer, compile the documents you already have (cadastral map, your own data like bank statements, employment contracts, income certificates, etc.) - sign the construction contract with the construction company with a clause that you can get out free of charge if the financing is denied (be careful here that the clause runs properly, e.g., fixed to a certain bank and preferably free of charge) - go straight to the bank with the documents from the construction company and close the deal (now you also hear the final interest rate) - sign the financing - buy the land