The first step is always to clarify the agreed security model and the status of the instalments.
Before your rights can be assessed, it must be clear how your advance payments are secured under the BTVG and how much you have already paid for which construction phase. Check the contract and instalment schedule to see whether a security registered in the land register with an instalment schedule, a bank guarantee or an insurance is agreed and who has been named as trustee.
Gather the contract, the instalment schedule, the payment receipts and the correspondence with the trustee. On this basis your position can be classified in the event of looming or opened insolvency.