Why building a house almost always costs more than calculated

We also had offers for almost everything beforehand, but unfortunately each would have been between 5 and 50% more expensive.

why... 50%?
what caused it to be so much more everywhere?

The interior doors were offered at a gross price of 250 euros. However, one ended up costing over 400 euros. The front door was included at 3000, but it cost 5000. We had estimated 20,000 for the garden without an offer, and unfortunately it became 45,000.

We often had offers and during the sampling we discovered many great things and eagerly said yes. Then the offer to be ordered came and bam, it was several thousand more.

On average, we are also well within the framework. Certainly, some items may be more expensive than planned, but others are cheaper than anticipated. Therefore, ±0 or we are rather below our previous calculations.

The interior doors were offered at a gross price of 250 euros. However, one ended up costing over 400 euros. The front door was included at 3000, but cost 5000. For the garden, we had calculated 20,000 without an offer, and unfortunately it became 45,000.

We often had offers, and during the sample selection we discovered many great things and eagerly said yes . Then the order offer came and bam, it was several thousand more

Ok, so the usual developer "trap"...
With a direct contract, you have the "sample selection" beforehand... well, as long as it was ok for you overall - 50k more just isn't something everyone can just accept.

We often had offers and during the sampling we discovered many great things and diligently said yes . Then the order offer came and bam, it was several thousand more

Here's the catch....yes-men are simply poorer at the end of the day.

We didn't build, but renovated. What we had previously calculated, we adhered to. We were even able to get some things a bit cheaper. But then many things came up that we didn’t want to do at all and therefore hadn’t planned for, but ended up doing anyway. Some unplanned things came up (especially in the plumbing area; new pipes, etc. and also a lot in the outdoor/garden area) and with some things we couldn't agree to after all, which we had previously thought "let's just leave it like that for now." Honestly, I seriously miscalculated the overall budget there. But even here you could say: if we had strictly stuck to what we originally planned, we would have stayed within the calculated budget. Basically, you could have left the garden as it was if necessary, could have replaced fewer windows, etc... but woulda, coulda, shoulda; as the saying goes^^. You want it to look nice and be finished as well. We should have better calculated with a 25,000 buffer.