Suppose I have a remaining balance of, say, 8,000 € at the end of construction (meaning the house is completely finished). Do I have to return the money, or can I keep it as savings? I have to prove all the money I spend to the bank with receipts! Therefore, my mind tells me that I cannot set the money aside! Or am I wrong?
It is highly unlikely that you will have anything left at the end. Here and there costs end up being higher than planned, there may be choices of materials where you prefer, for example, a more expensive shower panel instead of the originally planned shower, etc. If everything is actually completed and you still have some cash left, it will go toward the outdoor area. There are many nice things that will still come up. For example, anyone who has a cistern for garden irrigation needs a submersible pump and X meters of hose (or pipeline) costing 300 - 500 euros. You may need a fence (so the small children don’t run onto the street) – 8,000 € won’t be enough for that anymore. Mailbox with stand, 150-250 euros (for reasonable quality). Topsoil depending on quantity, several thousand euros, and so on. We had to submit receipts for payments – but not for 3.50 euros, rather for the invoice of the partial payment plus the construction progress report. I actually had to submit collected receipts when proof of the use of own funds was required. Since we covered smaller expenses (< 1,000 euros) from these, I had to send a thick letter to the bank at that time (which even included the 3.50-euro receipt from Aldi).