Fixed price commitment - price increase after expiration?

On what basis? According to the contract, the price was fixed for the agreed price, from then on it will simply be newly determined or adjusted based on the index. However, this will probably not happen on a daily basis. But certainly the price will be adjusted again if it takes further months until construction begins. You need to discuss the details with your home seller, that is, how long he guarantees the current price to you.

If construction start is significantly delayed on your part, he will surely adjust it again. Which, viewed objectively, is understandable.

That would clearly be to the advantage of the construction company, which suddenly no longer has to honor a price lock period on its costs. However, I will ask again how long the current price is valid.

Why should it do that? The granted price binding applied for the specified period, during which you then failed to start. Consequently, the matter is settled! I would definitely be surprised if the BU felt bound to the price for significantly longer than I say, about a month.

Why does she favor the contractor? She guarantees the client a fixed price for a certain period if he starts construction within that period or rather has the construction started. If this is not the case due to his fault, then the fixed price commitment becomes void and the price is basically reset at the start of construction.

So if the reasons why the construction did not start within the intended window lie with you, then it is your problem. You then failed to start construction within the fixed price period and now have to swallow the price increase based on the index. If you had started on the last day of the period, the construction would have been done at the agreed price.