I have a pool house that is 50 metres from the main house and although I have both on the same LAN, I’d really like to have only one Home Assistant (Blue) but access Zigbee devices in both locations.
How can I do this when the range is too far for Zigbee and it’s gardens inbetween with no easy way of placing Zigbee devices along that 50 metres (no power)?
Put z2m on a pi or similar in the poolhouse.
So I can have 2 seperate Zigbee networks via two usb sticks, one in each building? One in a Pi and one in the Blue?
Yes. Make sure you have a different mqtt base topic for each one, the default is zigbee2mqtt, change it for the remote one to something different. (The other computer can be anything, pi is just an example).
There are also zigbee coordinators with ethernet that you can place in the other building to set up a zigbee mesh there. It will probably cost much less than an extra pi + usb and will also be easier to find than a pi.
You’d still have to run 2 instances of z2m.
I’d also lean toward zigbee, but depending on the number and type of devices needed, might consider wifi devices.
As @Edwin_D suggests, an ethernet or wireless coordinator might be the best option. You wouldn’t need a Pi or equivalent for the pool house. Everything could run from the main house, potentially all on the HA Blue.
You haven’t said what zigbee solution your using - zha, z2m, deconz?
If you have the HA Blue, I assume it’s running haos. I haven’t tried running two z2m instances under haos. It’s probably doable, but not sure how much tinkering would be needed.
The simple, no fuss, no docker knowledge needed way I know would work “out of the box” with the Blue under haos would be the z2m add on for one coordinator and zha for the second coordinator.
I’ve got a ZZH MULTIPROTOCOL RF STICK (CC2652R1 - EXTERNAL ANTENNA) on the Blue (yet to set it up):
So would this be what I could just add as the cheaper than Pi option?
Wish I hadn’t bought a Pi within minutes of the first response 
- I’m sure I’ll find a need for it in the future!
Oops sorry. I’d prefer myself to do it the pi way and have 2 z2m instances.
If you want to sell it…
If you’re in the EU you have two weeks to send the pi back for online purchases. Choices, choices…
Not sure how legal this is in you area, but this guy used range extender antennas:
Long Range Zigbee Network - How far will it go? - YouTube .
Another method would be to setup a “point-to-point” connection using directional antennas (Example: Directional WiFi Antennas 2.4GHz / 2400MHz ) but you’ll need 2 routers that allow an external antenna + 1 router that is “in the view” of and “close” the antenna near your home.
I’m impressed you found one within minutes.