Would there be a way to setup something on a laptop (or mobile) that would allow you to roam around and see what the Zigbee coverage is like in your home and garden?
Perhaps a USB device that presents the signal strength to the network in an app?
I am just planning my new home and rather than buying load of unecessary repeaters, it seems sensible to walk around and map the areas that need addressing.
Similar to what you can do with Wi-Fi, before deciding where you need to add something.
I think your approach will lead to an unstable network. The more zigbee routers(repeaters) there are, the more stable your network will be. You should try using devices that act as router as much as you can (usually pretty much all main-powered devices are routers) and use end-devices (battery devices) only when its the only option (i.e. no way to connect to main power). The router devices will also help offloading the controller. Keep in mind that there is usually a maximum amount if devices that can be directly connected to a coordinator (referred as direct child). Having routers will ensure that you can have more devices in your network than the maximum direct child the coordinator can have, because some will be connected to routers (so not a direct child of the coordinator).
Having multiple routers = more than one possible route from a device to the controller band will ensure that the messages get delivered using an alternate route if required, for example, if the usual router is busy or defective.
I don’t know what kind of house it is, but it doesn’t take that much router devices to cover an entire home. If you really want to check the coverage, I guess you could use a spectrum analyzer but they are quite expensive. And keep in mind that not all Zigbee devices transmit at the same power level so if you get “coverage” from the controller or from a router at a specific location, it doesn’t necessarily mean that an end-device positioned at the same location will be able to transmit back to the controller/router from that location.
My advices is to buy a good and strong coordinator and to install it as close as poosible of the center of the home. Make sure there is no other 2.4ghz devices transmitting near the coordinator (i.e Wifi router). Choose you wifi network and zigbee channels so they dont interfere.
If using a USB coordinator, use a USB extension cable between the server running HA and the coordinator. Try to have at least one router device in each room you want to have zigbee.
What he said.
Signal strength changes all the time in response to traffic on the network and changes in the environment. It only takes your neighbour to buy a new wi-fi router and all bets are off. A robust network will probably have an over-supply of routing devices.
I have eight THIRDREALITY ZigBee Smart Plugs, Zigbee Repeater scattered around my house. ($10 each on Amazon). No Zigbee device is more than 3M from a router.
Thankyou all, very helpful 
There won’t be a problem with neighbours, closest is at least 500m away.
Is there an easy way to tell if a mains powered device is also a router? Is it an automatic function or need enabling? (For example SonoffNSPanel)
My HA is currently running natively on a old laptop, I bought a Sonoff v3 Zigbee USB device for that.
The house is timber frame with concrete block skin, so internal coverage should be relatively easy.
How can I get garden and garage (detached) coverage? (for some Zigbee relays, smart bulbs etc)
I added Zigbee bulbs to my outdoor lighting (10 bulbs) to bridge the gap to my detached garage. Worked perfectly.
Just to be sure: did you already see the ZHA Network Visualization?
This gives a rough impression of the Zigbee connections between all devices.
The signal quality is roughly indicated by the LQI values and line color coding.
To get there:
Settings → Devices & services → Integrations → Zigbee Home Automation → CONFIGURE → Visualization
Nearly all are, and it should be automatic. There’s a short list of devices that are not routers here:
Mains powered Zigbee devices that aren't routers
If you already have a device, the ZHA integration should tell you.
There’s probably something similar in Z2M.
If you want to keep track of signal strengths, it’s quite easy to do with an entities card:
It’s an interesting exercise - you can watch the values changing minute by minute and it may show up weak spots - but when you add an extra router it can take the network several hours to settle into a new configuration, so it’s not like an app to check BLE beacons (which I think is what you had in mind).
Just do your best to try following all these (low-hanging fruit) best practice tips and you should be fine but if not then do not troubleshoot before checking them again → Zigbee network optimization: a how-to guide for avoiding radio frequency interference + adding Zigbee Router devices (repeaters/extenders) to get a stable Zigbee network mesh with best possible range and coverage by fully utilizing Zigbee mesh networking
