Which smart home system to retrofit?

What would bother me if they run on batteries, optimal would be for example window contacts that run on enocean (maintenance-free)

Punctuation helps. Window contacts are ideally installed directly by the window manufacturer in the windows. Then they are invisible. The electrician then connects them to the wired (bus) system. That would be the proper solution. By Google, I have now found the Eltako FTKE window contacts. €50 each, but at least invisible when the window is closed. On Amazon you can find a picture of them installed.

Here in this forum, mostly only the basic things are communicated (mainly house construction as such and the "interaction" with the electrician/construction partner). But that's usually it because there are other forums that deal more in detail with the respective smart home systems.

You have already been shown the direction now. If possible, try to have the wiring done "star-shaped."

I would also avoid batteries. I currently have a Zigbee temperature sensor in every room. The batteries last surprisingly long. Still, it is of course annoying when you have to replace them. However, there are also switches with integrated sensors that can be retrofitted. You really can't do much wrong installing them if you haven't laid a cable after all. Even the Homematic IP presence detectors come with batteries. Allegedly 2 years runtime.

I never would have expected that the windows could be connected with the bus system.

So, to sum it up, if it is possible, one should always prefer wired smart home (with star-shaped lines, you can then connect any wired smart home to it?

You should only rely on wireless if wiring is not possible for cost reasons or if you don’t have a new building.

I talked to the builder, he is not familiar with it either, but Busch free@home has already been installed before (there is not much demand for it).

What do you think of Busch free@home? It is supposed to be an inexpensive and user-friendly alternative for private users to KNX.

but very very expensive (at least 10-15 THOUSAND EUROS) and very difficult to install and program
For changes, you often have to call an electrician (unless you have extremely good programming knowledge)

These are old wives' tales, mostly written by people who have no idea about home automation.

What do you think of Busch free@home? based on KNX even uses the same cables
Supposed to be cheaper and easier to program (thus aimed at private users)

You have already named the disadvantages. Very rudimentary functions and many limitations. The biggest advantage of KNX, namely manufacturer independence, is missing. Price-wise it is not necessarily cheaper than KNX either.

We had imagined having to spend a maximum of 5000€ now for smart home (preferably less) but if cables have to be laid, you have to do it immediately

Yes, that's exactly how it is. If you want a smart home, you need cables, and more than the standard installation provided by the general contractor... regardless of how it is controlled later. Because even sensors and actuators that communicate via radio suddenly need electricity for what they are supposed to switch.

What is available from e.g. KNX for window contacts that look good, maybe even invisible?

Pick whichever you want. 99% of all variants can be connected to the bus. No matter the manufacturer. Only if they use some proprietary radio-based protocol could there be difficulties, but even there a solution is often possible.

Practically all can be installed invisibly.

What would bother me is if they run on batteries, optimal would be e.g. window contacts that run on EnOcean (maintenance-free)

There are only two battery-free options:

Wired
EnOcean

motion sensors were not planned initially but could be retrofitted, right?

Motion/presence sensors are an essential part of lighting control in buildings. Furthermore, they can be integrated into alarm systems and other automations, e.g. shading. Many things only become "smart" when you give the system eyes and ears. Motion/presence sensors are exactly that.

Retrofitting is possible, but often ends up being a visual and functional disaster.

so to sum up, if possible you should always prefer wired smart home (with star-shaped cables you can then connect any wired smart home to it?

Please free yourself from the idea that there is "smart home for purchase" off the shelf or even from the bargain bin. Smart home is nothing tangible. It is always an interplay of several systems of the technical building equipment.

There is no wired smart home or radio smart home. But there are radio and wired protocols with which devices can communicate with each other.

you should only rely on radio if cables don’t work for price reasons or if you do not have a new build

Radio is not necessarily cheaper if you compare apples to apples.

(there is not such a big demand)

No, too many disadvantages and not really interesting price-wise. Only if you don’t know much and want to sell the builder something, then it is of course a good and simple system.

What do you think of Busch free@home, that is supposed to be an affordable and user-friendly alternative to KNX for private users

Supposed, but it is not.

I didn't expect that the windows could be connected to the bus system.

They are not. The window contacts are installed in the window frames. Reed contacts, for example. You do not connect these directly to the bus, but to a binary input. Only these are connected to the bus system. This way you can connect window contacts from any manufacturer A to a binary input of any KNX component manufacturer B. Or to binary inputs of any bus system from manufacturer C. Once again: First define for yourself what functionality, comfort, and aesthetics you really want and how important all of that is to you. I think with your budget of €5,000 you won't get very far. With that you could have the necessary cables laid (no idea what that really means for you) and do the rest with the mentioned retrofit solutions in the coming years.

Ok I have seen that smarthome is subsidized by kfw and bafa. Can I claim this subsidy if the developer does it for us? Would this subsidy be independent of the KFW 55 loan? (It has already been applied for)