Only if you do it the way I said you could with the relay and existing physical switches. If you wire it with momentary switches it will be fine
I haven’t experience with Sonoff but it seems strange to me that it could not know/report state of its outputs to report them to ha.
Anyway Shelly would satisfy your needs for sure. And its small fitting wallswitch boxes (european) together with original switches
Since the Sonoff is not the only that controls the light then it can’t know what the others do.
Just like if you and me are in charge of something, if I do something behind your back then you will not have a clue what the state is.
All switches are essentially blind, they only know what they have done themselves.
Ahh so you guys working with idea of powering up bulbs with multiple sources: sonoff and directly from another switch?
then you are right. But why to do that? why to not power up the bulb with sonoff being drived by all switches?
You aren’t reading what I’m saying correctly. The Sonoff will now what it’s doing and therefore HA, but if one of the old existing light switches is flicked, how is the Sonoff to know that? This is why the only REAL solution is to swap the old switches for momentary ones and wire them all to the Sonoff input
no, that was if you use it in a config with the old switches and just swapping out of of them for a Sonoff
this is what you need to do with momentary switches
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That is the correct solution. But if the wiring is not correct, it means pulling extra wires.
momentary is the best choice likely working with most wirings already built-in walls. And it’s my way to go.
However it’s possible with normal switches too if both L and N are available near lamp regardless switches state. it’s needed to powerup smart relay. It’s possible to achieve if 3 wires are pulled between switches and lamp.
As long as there are 2 wires from each light switch location that can be connected through to wherever the Sonoff gets installed then it will be fine. As you suggest, it could be an issue
btw if you have 2 wires from switchboxes to lamp, momentary will not work too, without pulling additional cable (unless it’s available next to light)
I would suggest using an ac triggered device at the switch that feeds the light, if a neutral is available or at the light itself, if a permanent active is available.
It needs to be able to use normal switches so are triggered by a change in state.
Use the old output from the 3 way to trigger the device.
I have used a m-elec stitchy switch for the same situation here in Australia
But the OP wants control from multiple physical switch locations. This is only really achievable using the method in my dodgy sketch. In that arrangement you can have as many switches as you want connected to the smart device
My 2 cents: use the simplest hardware possible setup, don’t complicate things and let Home Assistant do its best. You can 2way, 3way, actually Nway switching in HA (as long as they’re accessible in HA, simple toggling can be achieved in most of cases with individual relays that don’t need to be wired together or have exotic wiring at the switch).
It might take a quarter/a half of a second for HA to sync all entities in a Nway switching but it is not worth having a complicate setup.
It might be that the system fails when wife, kids, landlord, mother-in-law (well, actually, this might not be such a big problem 
) ) are alone in the premises with a hardware failure and a third party might need to have access to the system thus troubleshooting should be 1. simple and 2. safe enough.
No with my setup you keep the original ac wiring and send it to the smart module instead of the light. Then you connect the light to the output of the module
That doesn’t allow the smart module to energise the light if the light switches are off (ie: via HA), without adding additional wiring or changing the configuration somewhat. Also, depending on the device, the control input is ELV, not mains voltage (ie: my Aeotec Z-wave switches). Unless I’m misunderstanding how you have this wired. Could you please show me a sketch of it?
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Placing it at the light fitting where there is usually active neutral and switch wire is where I put mine.
This has been done to death so many times before, just search on here and you’ll find all the diagrams you need and links to videos if you prefer not to read.
In a nutshell, you can use a Sonoff with all your original switches and HA will know exactly what is going on, no point reinventing the wheel as there is no advantage to be gained over an already perfect solution.
The only thing that switches actual power is the Sonoff so HA knows exactly what’s going on, check any of the old threads on this as above.
I didn’t think that Sonoffs had a mains voltage switch input (gpio).
These work pretty well (re-flashed them using tasmota wifi hack) and fit them inside the connection box in the ceiling.
I use my ‘old’ switches as i found new (pulse-)switches too expensive (3 times as expensive as the dimmer itself
).I solved it simply by using the old switches with a piece of foam rubber stuck under it on one side
