Help with construction contract

It was not due to the ground conditions but rather the assigned height of the building in relation to the street.

No, a potential additional cost was not discussed beforehand. Instead, they ordered the gravel (did not obtain 3 quotes), installed it, and now I am supposed to bear the costs.

I have also read that the contractor must bear up to 10% of additional expenses themselves.

This is actually not quite legal. Whether he needs 5 or 50 supports, fills a puddle with gravel or a quarry is his problem. The risk lies with the contractor here.

That is incorrect. The contract for work and the construction service description is a standard contract. If deviations are made because, based on the soil report, the ground must be excavated deeper than the construction service description provides, the foundation slab must be reinforced, or the building must be raised higher at the client's request, then the client must bear the costs.

It was not due to the ground conditions but due to the assigned height of the building relative to the street.

No, there was no prior discussion about a possible additional cost. Instead, they ordered the gravel (without obtaining 3 quotes), installed it, and now I am supposed to bear the costs.

I also read that the contractor must bear up to 10% of any additional expenses themselves.

Are the 10% stated in your contract or how do you come to that conclusion? If there is no "cost cap," you would have to pay the entire expense.

I had it written into my contract or had a written side agreement from the developer that all earthworks and property connections are included in the price.

A savvy developer usually knows the ground conditions.

that in the case of a lump-sum (fixed) price the contractor must bear the costs for additional effort

The principle is generally correct, but in each individual contract it can look quite different again (depending on which specific assumptions are underlying it and under what conditions, since there also seem to be certain restrictions in this contract with which the contractor has initially protected himself). Furthermore, one must always consider the reasons that led to the additional effort, and even in the case of an agreement made in this regard, whether this agreement is effective at all in the individual case (this question is often relevant, for example, regarding agreements on passing on the ground risk etc.). There are quite a few aspects to consider in such cases, including the time at which the additional effort became known, etc., unfortunately it is often more complex than it appears at first glance.

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I had it written into my contract or had a written side agreement from the developer that all earthworks and plot connections are included in the price.

A clever developer usually knows the ground conditions

Also very clever of you :)

The contract for work relates to the house; in the construction performance description, the standard for soil excavation of 30cm etc. is usually specified. (specifications)

If the ground conditions require something different, it must be paid for by the client. Usually, such matters then go “through” the contractor, who has ordered the excavator anyway.
That’s how it’s handled, and that’s also what is stated in your contract.

The fixed lump sum price refers to the contractually specified house, the additional effort relates on the client’s side to the plot.

It’s a difference to build the same house with the same contract either on the Elbe with flooding or in Bavaria on a mountain. You have to pay for the extra effort from your plot; otherwise, it would be illogical. ;)

Best regards in brief

With the developer, everything is always included and a genuine flat rate.