Construction contract - What is regulated only after signing?

Make a list of everything you want. Have the contractor write a price next to it. The items he cannot offer or that are too expensive for you, leave out of the contract or take your list directly to the next contractor.

Everything that is not in the contract and ordered later usually costs a lot of money.

Where you want to be safe, include it. We only detailed the electrical work with the electrician later, it was about 20% cheaper. Same with the balcony railing. This is at least our experience, which mostly matches experiences from our circle of acquaintances.

I can only agree with everyone.
List potential items as cost factors and have them listed with the price in the contract.
Whether you then take them is another matter.
For example, we also asked about sockets but did not take them from the general contractor. The GC was happy about that. It's less work for him. From the electrician himself it was considerably cheaper [emoji2]
Tile size, tile format... two-way switches, SAT and LAN... ask about everything and position with a price. Possibly later commission it cheaper from the craftsman.

The chimney is installed by the chimney builder you trust – the chimney itself, of course, by the GC again.
A price shock can also be the walk-in shower, so position the price already now.

I would already include everything that could become expensive later in the contract, e.g. colored windows, exact window sizes, sliding windows, room height, tile formats, ventilation system, RC2 windows, surcharge for aluminum front door, blinds, prices for LEDs, walk-in shower, clay roof tiles, etc..

Best regards
Sabine

Whether to commission a gravel bed as a splash guard around the house

Better yes, why not. If you want to leave something like that open, you could include optional items with prices (possibly specific prices, e.g. per meter, etc.) in the contract. That way you don’t have to hesitate later and can already compare the whole package today.

Otherwise, the scope of services should be described as precisely and completely as possible, especially for the trades that cannot be left open. With the splash guard strip, you hardly take any risk. But if the contract includes 100 outlets and later you want 200, it’s hard to postpone that to later.

Make a list with wishes. Alone for your safety so that you also have a "final" price (which is not really final anyway). It is always better to reduce services (e.g., take a shower instead of a bathtub because it is cheaper) than to have surcharges. We had a good developer, whose prices for additional services were really OK, they just passed on the craftsmen's surcharges. My colleague, on the other hand, reported that there were horrendous surcharges and he sometimes hired external craftsmen for the glass door in the living room.