I’m more than happy to help you design a wired system if you want.
Take a look at this generic wiring plan to get you a starting point.
This topic might make for interesting reading if you’re looking for someone else’s experience
As for costs, then yes, KNX is expensive.
Thankfully KNX is not the only wired solution out there.
Velbus is a very powerful product that doesn’t have a millionaire’s price tag.
For example, both 4 channel relays have a retail price tag of €155 + tax
The price (the more costs) definitely depends on how much battle you might risk yourself.
If you choose the wrong system, you can end up spending a lot more time and probably more money in the end. And time will be filled with grief and frustration. Perhaps you can never get a decent system to run and you find yourself starting over and over again. This cannot be measured in money, in my opinion.
Would you object to me quoting you when I talk to new customers and installers?
“Never a truer word” and all that
Yes but usually the content in them is biased or not a complete picture, see my previous posts on some of the common smart globes and I am sure you will not have seen even a hint of anything less than perfection from those videos. It is helpful if you go into a purchase with a realistic expectation on what you will have if you spend X dollars/euros on each device.I am not saying smart globes are rubbish or don’t work, I am just saying they have their place and for some people they will love them and for others will want to throw them out and buy again. Whilst you are building it would be a shame if you did not take a serious look at wired solutions as later on the cost goes up to retro fit wires.
I agree that 99.9999% of people need to get a pro installer to design early on in a build, some of them will try to lock you into repeat business by not allowing you access to the software or programming tools so any change you want means a call out for them to program X device. Finding a good installer, the right gear and still be able to do the programming with openhab will be a challenge which is why it is great to see people like @MDAR helping out.
Go ahead quote all you want, Stuart!
I have been in this “game” as a regular costumor for enough time to say, things like these have to be right from start. Beside all the struggle, the worse of all is an angry wife… it can be worth a fortune not ending up in a situation struggling with wife and/or family, cause thats a lost battle even before it started!
Now that will remain for some time until you dug enough into HA technology to understand and actually plan it yourself, but in the end, you will have to do that anyway. Remember you want openHAB to control your home, there’s no company to program that for you.
Another problem with getting a company to do it is - besides what @MDAR already said and cost - that most are very much biased to some sort of tech. In Germany for example most installers understand “KNX” when you say “Home Automation”. They don’t know any other tech to combine that with, let alone a non-KNX central to hand control over to. They don’t know about (cheaper) alternatives but will try to do everything with and even plan the wiring for “their” tech (only). They cannot integrate existing or future household appliances, lights and entertainment media to come with a WiFi, Ethernet, Bluetooth or ZigBee interface (or it gets VERY pricey).
And sooner or later you will be having wireless devices because you will want to relocate or add more sensors, and more.
The best advice at this stage is to plan for a tech-neutral setup. That essentially means to centralize your wiring of devices to one or few wiring hub(s) in the first place - at least mission critical ones, preferrably all of them.
Then at a later stage you can deploy any tech (be it wired or wireless) or any combination thereof inside those hubs.
Remind you that any decision in favor of classic bus/loop-like wiring for KNX or Velbus will lock you into use of these - think what will happen if you have to exchange or complement any of these in the future.
There’s a number of more or less generic posts on questions like yours on the forum that I’d encourage you to browse.
Planning for a centralized wiring now will get you some time to decide on technology a little later.
I absolutely agree.
I’m openly biased towards Velbus because that’s my business, I understand the product and I’ve got plenty of very happy customers.
Obviously other companies / individuals will be biased in other directions, which is all absolutely fine.
What Markus says is 100% accurate, please plan for a technology agnostic setup, so that should a situation arise in the future where you want to start all over again, you probably won’t have to worry about changing cables.
Now you could take this advice to its natural next level and install flexible conduit so that your can pull through whatever cables you like.
This is (to my knowledge) the French and Dutch way.
I think I’d second this. Most people just getting into home automation are unlikely to understand the compromises involved when choosing one approach over another (the bulb versus switch debate above is a good case in point). However, if one does go with a wired solution with all wires leading to one (or more) central location then they can probably build in enough flexibility to grow into their system.
OK, “self, why?”
This is why, even for DIYers looking to retrofit an existing home with wireless devices, I recommend starting small and gradually grow a system over time. DIYers rarely have the experience or a full enough understanding of what they want to do or grow into at the start. And by starting small, the cost of moving to some other technology is much less because there hasn’t been as much investment. It’s far cheaper to replace a hub and couple of zigbee bulbs than to rip out 30 zwave outlets and wall switches.
While I’m usually down on bulbs, I agree they have their place. I have a new puppy right now. This means a trip to the yard at least once a night. I’d like to automatically turn on a red light on the stairs at that time of night but use a white light in the evening. A smart bulb is pretty much the only option. Though that bulb will be paired with a smart switch so the wall switch can still be used.
Thanks Rich
I knew I could count on you 
I didn’t want to do down KNX, but you are absolutely spot on. There are posts that pose questions about KNX (as an example), which is why I like Velbus so much
As Kim said in an email recently (which echos previous comments from customers) “Velbus just works”.
@Senne posted a similar sentiment on their topic recently.
@rlkoshak I’m guessing you don’t want a full on illuminated staircase ? 
I want just enough light not to kill myself but little enough light to not fully wake myself up. 
What kind of light does your new puppy prefere? 
I just checked- you can also now set a custom power-on behavior for your lights. So, they either come on at their previous state or they can come on to a pre-defined color and brightness. I’m not home so I can’t test it now, but I presume it includes a setting of default off.
I’m pretty happy with my Hue setup.
I designed & built my system using a combination of OH & Souliss. Souliss operates on cheap Arduino devices and has native support for RS485. My setup is pretty simple: all live wires for lightbulbs go to a box with a number of solid state relays connected to an Arduino Mega with an Ethernet shield & RS485 adapter. For the light switches, I used 6wire alarm cable to supply power (2 wires) and RS485 (another 2, leaves me with 2 spare :). In the switches there are Arduino Nano-s with RS485 adapters (fits nicely in a larger switch box). The Nano-s in the switch send commands via the RS485 link to the Mega to turn relays on and off; at the same time I can control the Mega via a phone using the OH app. Here’s my code for the Souliss devices (the OH part is pretty easy but if needed I can share that as well): https://github.com/Z0l/soulissconf
My logic with this design was the everything needs power and power comes with wires, so if the wires are already there you don’t need wireless for anything. I’m using a star topology for the RS485 instead of the ring but at these cable lengths it doesn’t really matter; also dimming is not currently supported on the light switches but technically you could do that with a dimmer switch that’s recognized by the Arduino (it will also need a relay or a bulb that supports dimming of course because the switch is only sending commands). You can prototype for peanuts and use exactly the same wall switches (with a pull down resistor) so wife/grandparents don’t even recognize they are in a smart home I’m using pushbutton switches, as they allow some future development (e.g. pressing them 2 times quickly.to turn on or off all lights on the floor) but that’s the only recognizable difference from a regular home light switch.
I have tried both sonoff and milight.
I do not recommend milight. They are proprietary and the wlan gateway is quite bad quality. It can also only control 4 lights. So you would need lots of gateways.
Sonoff works fine with tasmota firmware but there are some drawbacks. First you loose support from sonoff and can not go back to official firmware. Second the flashing and wifi setup is a lot of manual work. At last sonoff only produces a few types of devices. So you will need more than sonoff.
I am currently on the way of switching from Intertechno switches and rfxcom to zigbee. Zigbee has quite a few advantages. The setup is a lot simpler than wifi. The mesh network improves coverage when you add more devices. Zigbee is also provided by many manufacturers. I currently have Osram Smart plugs and light bulbs. Both are quite cheap (especially with recent Amazon offers) and Xiaomi Aqara sensors.
The zigbee devices are much less hacky than sonoff. They look great and are still cheap.
As a controller I use a CC2531 USB stick with a pre flashed firmware from ebay. I have no clue if this is the best option. Most devices work fine I only have problems with a osram motion sensor which should be supported but does not work and one type of Osram rgb light bulb that is not yet supported.
At the moment I use zigbee2mqtt and node red for the rules. That works very well but lacks a good UI for phone and tablet. For the rest of my system I use openhab and habpanel which provides a very nice table UI. So I probably will switch to zigbee via openhab.
Zigbee2mqtt supports a lot of devices: https://www.zigbee2mqtt.io/information/supported_devices.html. The list in openhab is smaller https://www.openhab.org/addons/bindings/zigbee/.
I hope the switch to openhab zigbee binding works with more than the listed devices. When I got my system working on openhab I can write about it on the forum.
You should also consider to use a wired control. If your house is not yet built the difference in cost is a lot smaller than later. It is not easy to move a non automated wiring to zigbee or anything else without loosing a lot of comfort. For example you want your light switches to still work.
While the idea is sound, if you were starting again I’d say to run cat5 cable for flexibility. Not only would it allow longer RS485 runs, it can be used for several other technologies if things change later. Even alarms 
Probably yes, but back in the day the alarm cable was both free and shielded (it runs next to the mains) so it seemed a logical choice. STP is still expensive for cat5.
Thank you for making this blunt observation.
Please accept my apologies, like so many “professionals”, I’ve completely overlooked the fact that people do need to start at the beginning.
To make amends, I have created this short (if 16 minutes is short) video that covers the very basics of a wired control system.
Obviously I have used Velbus hardware, but the basics apply to most modern digital control systems.
Separate thread on the installation of wired systems
Pro or not Pro… I would recommend everyone to start from the beginning, everytime
I built my new house in 2015/16. I had no idea about smart homes. My Electrician suggested to use KNX, even if it’s quite expensive. Because the ability to change and upgrade later is very good. I am very happy now, that I have this system! You can still make the same parts DYI, but this is much more work. Also the power consumption of the KNX is very low. Nevertheless I combined it with Openhab for visualization and other systems like Z-Wave. The big + of KNX is, that it is one bus, it is wired! and if the LAN fails, the KNX is still working. It it really easy expandable and changeable. Rules are managed with openhab, but if that fails, its ok.
I also recommend Z-Wave, but not for long distances!
ESP-easy for senzor data over Mqtt is nice!
If you are looking for a cheaper, non-cloud solution, there is also Loxone. Check it out.
But I strongly recommend a wired bus solution independent on the LAN!
Just for reference, I wouldn’t describe Loxone as cheap anything.
It needs a central control unit, that I understand has a base price of around £450.
In contrast, Velbus doesn’t need anything other than the modules you want to use.
A minimum setup ‘might’ be.
(All prices quoted at full retail, Inc tax)
A USB interface module - €121
4 channel relay - €188
2 button glass panel - €100 (plus back box and mounting frame ~€4)
15v PSU - priced to suit current requirement