Is the cost estimation by the architect realistic?

if each trade reviews the overall plans in advance.

But you did not receive any execution plans until service phase 4. Only rough sketches and a plan for approval. Many details are missing, of which you have no idea as a layperson. That then causes a lot of rework, which is extra expensive and massively delays the construction. And for every damn thing, you have to answer questions spontaneously on the construction site. I hope your employer is okay with that?

A plan for an approval is simply much less detailed than an execution plan. As already mentioned, you can familiarize yourself, but it is very time-consuming and prone to errors (especially if you are not a specialist). It is certainly advisable to have the execution plan made by a professional. Especially because as a layperson, you often don't even know what is important and what is rather unimportant.

Can you give some examples for this? The foundation has to be specified by the shell builder, i.e. what he wants or better yet, he does the excavation himself.

Foundation: with us, it was determined by the soil expert together with the structural engineer and supervised on site. Problem areas are especially the interfaces between trades, for example with our first house the sealing of the balconies (which was not properly specified by either the shell builder or the tiler), electrical planning and shell builder (cable routing and planning), plaster and external blinds, to name just a few problem areas – and this despite having an architect. We are currently planning/building again and I am glad that our current architect only does the tenders after the ENTIRE detailed planning is finished. But that is just my opinion.

Can you give examples for this? The shell builder would have to specify what he wants or better yet do the excavation himself. Otherwise, I can hardly imagine where there could be massive problems if every trade looks at the overall plans in advance. But maybe I'm just a bit too optimistic. The shell dimensions are already given. From performance phase 5 onwards, I only see massive potential savings with the architect and am currently wondering if I really need all of that.

Hardly anyone actually thinks and works properly for you. If you have the ability to get familiar with all trades enough to specify how things should be done, then that's possible. But very few people have the time and opportunity for that.

We are building with individual contracts, but with a construction company that did the architectural planning and also the shell construction. Even there, there were communication problems. The idea of also being responsible for that sends chills down my spine. Although we took over the construction management after the shell construction.

The foundation must be specified by the shell constructor, what he wants to have there or he should preferably do the excavation himself.

You won't get as much advice as you imagine. If you then request a quote in this way, you will get the most expensive or simplest option, but not what you actually need.

But most of the requirements already result from the structural engineering and from some energy-related specifications, for example [KfW55]. What options does the shell builder then have to sell to me expensively or cheaply? With 3-4 offers, a rough direction should then already emerge?!