Bus with sub-branches is probably a good idea and will need least cables.
Do an open ring bus (both ends in the breaker cabinet, only one connected) with sub-branches then you can eg. remove a wall where your bus was going once and just connect the former unconnected end to keep everything working as before.
My home automation is wired just like you envisage.
I was in a retrofit and so I have used my existing mains wiring and supplemented each light switch with a switched live back to my distribution panel. Actually this comes from ceiling rose so no switches had to be replastered. These go onto relay(s) that are all switched by one control signal that switches between ‘automated’ and ‘traditional’. I can therefore revert the house to an absolutely normal wiring scheme should I vacate.
I actually use Schneider C-Bus (Clipsal) and am very happy with that choice but if I was in a new install now I would probably go KNX or maybe Loxone. I personally avoid any RF based devices if possible. My C-Bus is integrated with HA via MQTT.
KNX is expensive but very solid and there are many installers. The software licence (for configuration) can be expensive too. Bear in mind if you get an installer involved they will own your project file. They may well give this to you but invariably they then disown the whole project as soon as any modifications are made and charge again to rectify any errors you have introduced. It is surprisingly easy to destabilise a large KNX configuration.
There is currently a discount for HA users 
have a look at the integration documentation for a link.
You get a significant discount once you complete KNX’s eCampus online training too. Not sure if you can stack discounts? Im toying with setting up a smallish KNX install here to familiarise myself. ETS6 home should suffice.
KNX is not as prevalent in the UK as it is in EU.
There are also up to -30% or even -40% discounts (at least for Professional - not sure about Home) every once in a while (~4 times a year) on knx.org
Definately go with KNX. It’s more compact, widely supported and very flexible, there are all kind of options available. And it’s growing fast, a lot of activity around it.
Going with standardized KNX-solution also adds value to your house if you sell it into future. Electric stuff typically needs some qualified person to do repairs and installations, they may know KNX but don’t know some odd and rare solutions. That will reflect to your house value.
Juha
Hello,
i am in the same situation ie planning old flat renovation 75m2 (3 rooms) but completely confused. Currently running complex zigbee/zwave HA setup.
Not clear what benefits KNX brings - end point devices ie switches? sensors?
also for a lightning (deltalighs) does it make a sense for such a small flat use dali? and if so who / how design the lights… specific lights, their placement, scenes?
i was assuming there might be some blog/ howto / similarly as for example if someone is playing with NAS/hypervisors… but in that area, i cant find anything specific… mostly so much generic descriptions … 
thanks!
thanks!
Hi, that sounds like a cool project. I completely gutted our condo 79m2, 3 rooms and installed KNX and DALI throughout. I chose it at the time because of the standard and because it forms a solid basis for a smart home.
I had previously set up several smart homes using Shelly, Zigbee, and other technologies. Inevitably, with growing complexity, you constantly have to troubleshoot. Whether it’s changing batteries here or debugging a connection there. It’s not a big deal if you’re interested in it, but you have to be aware of it. Also, Home Assistant as a central control system is great, but if the server goes down, you’re stuck, and in the worst-case scenario, you have no light or heating. You can protect against this, but it just adds more complexity.
KNX, on the other hand, is its own bus system with no central components (apart from the power supply, which you can make redundant). This makes it extremely robust against failures. KNX doesn’t necessarily have to be expensive either, especially if you get involved yourself and can handle the parameterization.
Personally, I think KNX is the best thing you can do to lay the foundation for a smart home. Of course, I also have a few Thread and WiFi components, as it doesn’t always make sense to run a bus cable to every corner, and some things simply don’t exist for KNX. Ultimately, everyone has to decide for themselves. If the electrical wiring needs to be redone anyway, it would be a no-brainer for me. With good existing electrics, you have to consider your options. For example, I am currently working on a project where rooms are being converted to KNX one by one. That’s possible too. What I also find very appealing is to link actions from Home Assistant, like “vacuum the kitchen” for the robotic vacuum cleaner, to a normal KNX switch.
For us the decision for DALI was, that we wanted to have fully integrated tunable white lights that can do a HCL curve by themselves without HA. Also they should have a control wire and not be wireless. We pretty quickly landed on DALI and a KNX DALI Gateway with HCL function. Our lights are mainly 24V CV lights with Lunatone controllers.
And if you decide to go with KNX, look around! There are many manufacturers on the market, and some really cool products available. KNX is a system where you invest a lot of time and brainpower at the beginning, but then it runs for decades. You can always add Home Assistant on top of it to handle more complex automations, but it’s not strictly necessary, and you can connect multiple smart home systems to KNX however you want.
Finally, a point I believe is very important is the sustainability of the system. Anyone who relies on KNX can, in a pinch, get help from any certified partner if you can no longer manage the system yourself (for whatever reason). The same applies to selling your property. If only you understand the system, no one will want to buy it at first. Sure, you can probably revert it, but over time, it will only become more integrated.
Documentation is very important and is key to a successful deployment of not just a home automation system but everything in your home. You should have a hard copy binder. It would contain all of the documentation that came with your components. It would have a diagram of how everything is connected. It would have a description of what every switch does, particularly multi button switches. If some of your documentation is a link, a printed copy the page to which that link points (Your abode is going to be there for many, many more years than that URL is!)
This is no trivial task, but one that everyone, me included, avoids like the plague. You’ve heard of RTFM? In these times it is, WITFM (Where is the … Manual). No matter what you think, it’s not obvious nor trivial.
The documentation for HA is pretty good, but try to remember your experience coming up to speed with it. You have to document that. You hit the nail on the head with
This is what needs to be documented!
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When setting up a KNX System the bare minimum is:
- A plan of the distribution box, where all cables are documented (where they go and to which actuators they are connected to)
- The KNX Project File that documents addresses and location of all components and the functions of each area. Also encryption keys are stored in that file, hence why its password protected.
Additionally a floorplan where each receptacle, light, sensor and switch is documented and numbered (as well as all the cable routes) is also commonly done, but that information can also be embedded into the project file (minus the actual floorplan)
The paper files are usually stored in a binder directly in the distribution box with a usb drive containing the file.
Of course there should be a second copy offsite.
KNX makes it pretty easy to document all that since there are tools for that. Of course you can use the same ones with HA but as it is a newer system with ever changing features and UI it might be more difficult to come up with a standard way of documenting it, so someone can come in, change a specific thing, document it, and get away again without knowing everything about the system at first.