We are planning our smart home in the single-family house

First of all, congratulations! Seriously! You have understood the basic idea of SmartHome better than 90% of all others (including many manufacturers of supposedly smart products)!!!

Thank you very much for your honest feedback!

Most of your wishes are ABSOLUTELY sensible! The only thing I would doubt is door sensors if you mean interior doors, I really see little use there (but I assume you don't mean interior doors, although I have seen/heard of that), but you can make use of it yourself, though that goes quite far.

Indeed, only front door and patio door are meant. Nothing is planned for interior doors.

The heating control is questionable, but that depends on your heating system, whether it is worthwhile or not.

A Tecalor (Stiebel Eltron) THZ 5.5 eco is to be installed. KNX connection is basically possible, and heat pump + central residential ventilation in one. I think we will hardly adjust the underfloor heating, but room temperatures should be digitally controllable (probably the actuator at the heating circuit valve should be enough for that, I assume?). Since the heat pump also includes ventilation and possibly passive cooling, I already see more interesting use cases here, which is why I would consider a general control. Exact use cases still need to be defined. Example: If at least one window is open in the morning and outside temperature < inside temperature, then extract fan at max to draw fresh air in from outside (something like that…)

I would also like to question the price comparison from Homematic to KNX. Are 5-10k purely component costs? An electrician who installs and programs/parameters it will also charge you money here.

Yes, pure material costs. With some experience, programming can be done yourself with these parts. Installation in the larger flush-mounted boxes should probably not be a cost factor (I suppose?).

In (for me it is a) standard work for home automation with KNX etc. there is an example calculation which, as far as I remember, covers a similar or rather larger scope regarding wishes with KNX and DALI, where costs of about 15k are stated in my edition, although the prices are a few years old and the scope still has to be properly compared and not recalled from memory. The initial costs for KNX are of course high compared to most other systems. But the more you network, the cheaper it gets overall.

15k€ would be completely within my budget. I am more worried about costs in the 40-50k€+ range, in which case the whole project will probably be dropped for budget reasons (my better half won’t agree either).

So, that should answer (1) and (4).
(2) Think about keyless entry, mailbox monitoring, calendar for garbage or other topics, power measurement for circuits like washing machine or refrigerator to detect if finished or defective or similar, calling up scenes (sleeping -> no doorbell, showering -> towel radiator, ... just suggestions not a must!!!

Exciting, I will add that to the wish list!

short addendum, the 15k€ were also component costs (including 1000€ for the software itself) for programming KNX yourself and thus without an electrician -> was only meant to draw the comparison to your Homematic price indication. But you definitely have to compare the scopes here! The calculation example included dimmers, switching actuators, roller shutter actuators, current measurements, window contacts, temperature sensors and controllers, and so on and so forth. That is already a reasonable scope for a single-family house. Maybe (but I cannot judge) more or more complete than in your estimate for Homematic. I don’t know how deep you went into it. I installed Homematic for my parents for the roller shutter control, so I don’t want to speak badly of it. Works flawlessly and for pure roller shutter control it was definitely cheaper and less effort than KNX. But I want to point out that the more you implement, the smaller the price difference becomes and depending on the focus and wishes it can also lead to a sign change, or also not.

A Tecalor (Stiebel Eltron) THZ 5.5 eco is going to be installed. KNX connection is basically possible and heat pump + central residential ventilation in one. I think we won't adjust much on the underfloor heating, but room temperatures should be digitally controllable (the actuator on the heating circuit valve is probably sufficient for that, I assume?).
Since the heat pump also includes ventilation and possibly passive cooling, I already see more interesting use cases here, which is why I would consider a general control. Exact use cases still need to be defined.
Example: If at least one window is open in the morning and the outside temperature is < inside temperature, then exhaust to max so that fresh air from outside is drawn in (something like that...)

There is a widespread opinion in this forum regarding individual room control and heating actuators (I might add that I agree with this opinion). The usefulness is particularly questioned in combination with heat pumps and underfloor heating. Highly insulated houses with slow-reacting underfloor heating systems and low supply temperature due to the efficient operation of heat pumps usually do not allow significantly different room temperatures anyway. Therefore, the flow rate and supply temperature for the entire system can rather be adjusted (but this must be clarified with the heating engineer). This way, dispensing with switching actuators and control valves would be possible. But then it definitely no longer works on a per-room basis.

Yes, pure material costs. Programming can be done by oneself with some experience on those parts. Installation into the larger flush-mounted boxes probably shouldn't be a cost factor (I assume?).

15k€ would be totally acceptable for me. I'm more worried about costs in the range of 40-50k€+, in which case the whole project will probably be dropped for budget reasons (the better half won't agree either).

I just wanted to first try to draw the comparison so that apples are compared with apples. So comparing material with material or a complete system including installation with the same. To be fair, it must be said that familiarizing oneself with KNX parameterization is definitely more complex than with Homematic, and that should not be underestimated. So a KNX system from an electrician would likely land in the range you mentioned, which I really find hard to estimate. But a Homematic system ready from an electrician won't stay at 10k either.

There is a widespread opinion in this forum regarding individual room control and heating actuators (by the way, I share this opinion). The usefulness is especially questioned in connection with heat pumps and underfloor heating. Highly insulated houses with slow-reacting underfloor heating systems and low supply temperatures due to the efficient operation of heat pumps usually do not allow significantly different room temperatures. So rather the flow rate and the supply temperature for the entire system can be adjusted (but this must be clarified with the heating engineer). Thus, the omission of switching actuators and control drives would be possible. However, this then definitely no longer works per room.

How should this be handled in such a case when we want 19 degrees in the bedroom but 22 degrees in the living room? That should be possible, or am I misunderstanding that?

I just wanted to try to make a comparison so that apples can be compared with apples. So material with material or the complete system including installation with the same. To be fair, it must be said that getting familiar with KNX parameterization is significantly more demanding than with Homematic, and this should not be underestimated. So a KNX system installed by an electrician can really end up in the range you mentioned, which I find hard to assess. But a Homematic system finished by an electrician also will not stay at 10k.

Understood, here I need to read into this a bit more deeply to realistically assess how much of the KNX programming I can manage myself and where I need the help of an electrician.

Maybe a silly question, but is individual room control of the ventilation even possible with a central ventilation system (Tecalor THZ 5.5 ECO)? According to the operating manual, you can only choose between levels 0-3 of the ventilation, but I assume that this always applies to the entire house? I would like to be able to control the supply air and/or exhaust air (bathroom) specifically. Is something like that possible?

How should one handle this in a case where we want 19 degrees in the bedroom but 22 degrees in the living room? That should be possible, right, or am I misunderstanding?

You can plan that as part of the heating design. Basically, 19 to 22 degrees in a new building is more than barely achievable. Then rather sleep with the window open to make it cooler…