ᐅ Is it possible to retrofit a smart home?

That is not entirely true for Homematic. Most systems operate even without a central control unit (lighting, heating, shutter linkages). This means that you can control all devices without the central unit, and time schedules, for example, are stored locally on devices such as thermostats. Automatic reactions to certain events, of course, no longer work without the central unit. In my experience with HM over three years, I only had one outage of the CCU (because I overloaded it through unfavorable programming). Otherwise, it sometimes runs a bit slower but is absolutely stable. The effort required for programming should not be underestimated, but this probably applies to most systems. However, it is extremely flexible and can be combined with many other systems (e.g., FS20, Philips Hue, Sonos, all devices controllable via IP, etc.).

However, I must admit that I do not know Z-Wave very well, as it was still in its early stages and quite limited three years ago when I chose HM. This may have improved significantly today. For me, a feedback channel is absolutely essential, because I don’t want to come back from vacation only to find the heating and lights still on because the signals did not go through. This is even more important when controlling security-relevant devices (front door, garage door, window openers, shutters, awning control).

KNX is certainly the premium solution, but it is only partially suitable for retrofitting and is ultimately the most expensive system. If I were building new, it would definitely be my first choice, but only because I know exactly what I want to control with it and that there is a corresponding benefit. For those who want to test how much added value it provides, starting with a modular system is recommended.

and the best modular system is:

*drumroll*

exactly! KNX...

but only partially suitable for retrofitting. For this reason, I recommended Z-Wave because it is well suited for retrofitting and can do many things that are otherwise only possible with KNX.

86bibo schrieb:
If I were to build new, that would definitely be my preferred method, but only because I know exactly what I want to control with it and that the corresponding benefit exists.

May I ask which applications you are currently using and would consider reusing?
I am currently dealing with this topic myself and trying to figure out which areas I find useful for us.

86bibo schrieb:
KNX is ultimately the most expensive system.

That is not entirely correct... with a sufficient number of devices to be controlled and the desired functions, KNX can actually be more cost-effective...

It is only more expensive if you want to dim three lamps and operate five roller shutters.

sonnenkind80 schrieb:
In the end, all of these solutions have one problem – they are centrally controlled. Meaning: If the central system fails, nothing works anymore.

This is not entirely correct, although I understand your point.
If the central system fails, for example, I can still turn my Philips lights on and off worldwide via the Hue app or access the security camera remotely through web access.