Is Smarthome KNX automation possible based on the floor plan?

We will see. However, the general contractor already said for the sanitary objects that nowadays everyone compares prices on the internet anyway, so they set corresponding prices (and if someone still wants, they can buy all bathroom objects themselves and the sanitary company will install them). Of course, just to prevent further tedious discussions here: we will do the electrical as well as sanitary sampling before signing the contract and include the whole thing in the contract.

By the way, for example, it would also be sufficient if the electrician only installs the flush-mounted bus coupler per room and we can buy the sensor/button ourselves and install it (if his price here does not come close to the internet price).

You can also take out individual trades. I also have an electrical master technician in the family.

Well then, I'm curious to see how your flag will still turn.

You really only need bus couplers if there is a tactile sensor/PM/motion detector installed from the start.

If it is only prepared initially, just an empty box with a cover is enough, in which the green cable simply lies. Later, if needed, you can put whatever you want into this box.

The absence mode you are talking about is a simple central function, also called a scene. For this, you simply need a double push button at the input, with which you can call the scene "present" or "absent" in the simplest version. Once you have done this, a sequence of actions is executed, such as, for example, blinds set to auto, lights/outlets off, etc.

Dimmable lights are quite good... for this you just have to wire all the light points with a 5x1.5 NYM cable, then later you can decide whether they should simply be dimmed or controlled via DALI or something completely different.

Heating/ventilation is a topic of its own; here you have to look at whether it even makes sense to integrate the systems into the BUS. The gateways are often disproportionately expensive. However, you can do it later in an emergency. Again, simply lay BUS and possibly NYM cables.

I would generally install window contacts on all windows (have them installed); these are simply necessary for various logics. For example, so you don’t end up outside in your underwear when the terrace shutter goes down.

You probably won’t find a washing machine etc. with bus integration, or the price will be so exaggerated that it’s simply not worth it.

Star, tree, open ring—however you lay the green cable, it will be correct. It just must not become a closed ring.
Imagine it like this: in the ground floor, the green cable comes out of your utility room and runs consecutively through the rooms... through every box (in a 2, 3, or 4-way combination, of course, it is enough to equip only one with BUS) until you arrive back at the utility room. This end is left open... so this would be a large open ring.

You can also do open rings room by room... overall you would then have a star...
Or you lay a main cable from which the open rings branch off as spur lines for each room. That would be a tree...

You see, it doesn’t matter at all... there is only one rule: No closed ring.

The advantage of open rings is obvious. You always have two ends in the utility room. If, due to construction or at some later time, the cable is pierced, for example, it’s no problem... you simply connect the other end and the room has BUS everywhere again.

One more note about the bus couplers: These depend on the manufacturer, you cannot just connect any devices. Moreover, most components now have integrated bus couplers, so you just install them in the box, connect the two wires to the terminal, and that's it. So, if you want to do it yourself, just leave the box open with the cable and you buy your devices yourself.

But why are you actually dealing with such details now? At the moment, you believe that you don't need this anyway? Or are you not convinced by the scenarios? I often have the impression that you want to see the end of the path even before you have set your first foot on it.

Is a central unit necessary and are there good open source solutions like openHAB or Smarthome.py or are proprietary solutions still superior for this?

Found a few things that I would probably need:

Weather station:
MDT SCN-WS3HW.01 Weather station Home
258 EUR

16-channel switching actuator or 8-channel roller shutter actuator:
MDT AKU-1616.01 Universal actuator 16-channel
263 EUR
(3 Venetian blinds; 16 roller shutters = 3 of these things and then exactly 10 switching actuators are still free = 789 EUR)

Bus cable:
EIB-Y(St)Y 2x2x0.8 Tr.500 EIB-/Bus cable
133 EUR per 500 meters
(Example 2,000 meters = 532 EUR)

Presence detector:
MDT-SCN-P360K4.01 Presence detector
110 EUR
(Hallway upstairs + stairs = 1; hallway ground floor = 1 = total 2 = 220 EUR; but I also read recommendations to put a presence detector in every room, then it would be 13 = 1,430 EUR)

Window contact, e.g. Gira: 15 EUR
(20 windows/doors = 300 EUR)

MDT STC-1280.01 Bus power supply with diagnostic function, REG 1280 mA
260 EUR

Dimming actuator:
MDT AKD-0401.01 Dimming actuator 4-channel
268 EUR
(16 lamps, of which at least 9 dimmable (living room, dining room, kitchen, children’s room, bedroom, dressing room, work area) = 3 or 4 dimming actuators = 804 - 1072 EUR)

All lights, inside and outside, are wired in a star topology with NYM-J 5x1.5?

Do the switches also need power or only bus?

Do the buttons also need power or just the bus?

The power comes via the bus. You then also need an IP gateway and a 30V power supply with appropriate capacity (depending on the devices). And don’t forget the €75 for the Lite version of ETS, the Pro costs around €1000, unless you get vouchers for one of them. But again the question: Why are you dealing with these details? You really seem to have too much time...