How to factor in variables of construction financing?

Can anyone here give me an assessment?
Do you calculate, for example, expected average costs + 10% buffer or the maximum expected costs + 10%?

I would definitely calculate with 1600€/m². However, even with the actual offers, you have to compare the details yourself. What is included, what is not included, what needs to be upgraded. With the earthworks, there are always unexpected costs (additional inspection shafts, access road, leveling, rocky substrate, etc.). The same applies to electrical work + bathroom. In the end, it will still be a bit more.

What about garage, carport, garden shed, outdoor facilities, paving work, etc.
For a kitchen, 10,000€ is also "just" standard. Of course, sometimes you can get a bargain or buy at Ikea (although our Ikea kitchen in the rental apartment also cost almost 8000€ without installation), but you really have to be strict and keep everything standard. Slightly better tiles in the living area cost quite a bit of money. On the other hand, you will be annoyed for 40 years by poor tile quality.

Garage/carport/garden shed we do not need. Are exterior facilities, paving work included in the 1600 €/m²? It sounds like it now. Because I have listed them separately (in the ancillary construction costs) and have already calculated them. Is there a breakdown anywhere of which costs are included in this 1500/1600 flat rate and which ones you add on top?

I mean, it doesn’t help me now to finance 300k (or not, because we come to the conclusion that it exceeds our possibilities) and in the end not use 40k of it because my calculation was actually pretty accurate.

Would be very grateful for further assessments!