What’s the distance from the shed to your house? You could potentially have a Xbee3 as a router at both locations. I’ve heard of xbee3s repeating zigbee as far as ~120 ft.
Alternatively, you can have another Hubitat in the shed and link the two Hubitats using Hub Mesh.
So the shed is about 23m (75 feet) from the house. I have just taken a smart socket from the house and moved it to the shed. It could not connect (status: not present), so I guess it is too far for a direct connection.
Try to locate an interior Zigbee repeating device as close as possible to shed's plug (and at the same height, if possible). I have a Zigbee battery operated sensor outdoors (about 80 feet from a repeater located on the ground level garage of my house) and it has been working without issues.
I have a shed which is 95 feet from my house (rear porch). It has steel panels on the walls and roof. But I have a few glass windows and I put both a zigbee and zwave 120v repeating device hanging in the center of one of the windows. Has been working great for a couple years. I also have two repeating devices on the rear porch.
If you are comfortable with a little soldering, DrHaas has a thingshield replacement that acts as a Zigbee repeater Forum Link
What makes this device so attractive for your situation is the Zigbee board has a connector for an external antenna.
You can also directly connect contacts and voltages.
Just a thought 
I haven't done this, but I think you can:
Add a switch to your powerline adapter.
Plug in a Hue hub, use Hue compatible sensors/lights in your remote shed.
Use one of the HE/Hue integrations to control/automate everything.
Used V1 hubs show up real cheap on local buy/sell websites, probably less than the Sonoff one.
Nice idea. I know the built-in integration works with the V1 hub. And the version of CoCoHue I tried a long while ago did as well. So I'm pretty sure the built-in integration will still work with the V1 hub.
However, because the V1 hub hasn't received any firmware updates, I'm not sure if the new Advanced Hue integration or the new version of CoCoHue will work with it.
Really like this idea. My hotspot already has Ethernet connections so that makes it easy.
Any advantages of using a V2 hub as I can see some of those being sold for a reasonable price? @aaiyar would they work with the integrations you mention. BTW I not looking to do anything too complex just report on contact sensors, turn on switches etc.
Since the V2 hub has not been "obsoleted" by Philips, I would think they still receive firmware updates and will work with @armand's Advanced Hue Bridge Integration. I have linked to his integration below. I think the possibilities it opens, especially in a situation like yours, are quite amazing. But I would definitely recommend you read the release notes before making your purchase:
Personally, I would leverage the poweline ethernet to host a 2nd Hubitat. I would use that device to talk to all your garden devices, and use the Hubitat Mesh to link the hubs, and expose your devices. I use the mesh and I have power line ethernet too, but what I do not do in this example, is to run the hubitat through the power line ethernet, because I serve both hubitats at the same room. I can speak from experience that the power line ethernet devices are a true bridge device, so there is no filtering. As long as both power line ethernet devices are on the same leg of the the transformer, there should be no problems, and the devices will perform as if they were native to the main hubitat.
I am always weary of products like the ZBBridge, because Zigbee is not ethernet based, and cannot work over ethernet, except to be encapsulated into an ethernet frame, which then means that these are non-standard, and require specialized software to interact with the zigbee devices.
I read the manual. They call it a bridge, but it is not. For this to be a bridge, you would need two of them, and they would use ethernet as the mechanism to span the bridge allowing all zigbee devices on one side to see the zigbee devices on the other side. This is technically a gateway. It allows you to use one protocol (wifi) to communicate with device on another protocol (zigbee). The way this works, is that you install their app on your phone. Then using the app, you join the device to your wifi, then pair all the zigbee devices to their module. Once paired, you can control then using their supported wifi devices, which include Alexa. This type of gateway is the same thing as our smart home hubs. So why they call it a bridge is beyond me. IF there is a hubitat interface for the sonoff bridge, then it would be an inexpensive way to accessing those devices. Otherwise, to keep it simple, I recommend some Zigbee range extenders, or just using a 2nd hubitat.
If you do decide to go with a Hue V2 hub/bridge check your local retailers for starter kits. I've never set out to buy a Hue Bridge, but there's several V1/2's in my spares box from deals on starter kits. 
The total price of the kit, with bridge, has been lower than what the bulbs/buttons/strips would have been individually.
@armand Thank you for that info. The one thing that is stopping me from 2nd Hubitat (which I agree would be the best solution) is price. £135 to control only a couple of devices is a bit steep especially when I can spend £10 on a V1 Hue Hub or £30 - 40 on a V2.
You mention zigbee range extenders. This may be a another solution using the the IKEA Tradfri repeater which is c£20.
@Ranchitat that is exactly why the V2 hubd are cheap new ones as people have bought the starter kits and are selling them off!
You know, if those devices work on Hue, my hue integration can expose them as sensors to Hubitat. That may be the solution. I don't have a driver for contact sensors, nor do I know if hue can use them, but if it does, I can can create an integration for that too. There are some dimmers on hue I cannot add to hubitat, because of the limitation in the current API (v1), but that generally doesn't apply to simple sensors. It will provide you with near real-time updates. I say near real-time, because Hue it a little laggy in sending the events out their event stream, and then hubtitat has to consume them before my code can do anything with the data. Typical delay is less than 2 seconds, often times less than 1 second.
In my experience in a similar situation (actually almost twice the distance). This is all within the capabilities of well placed repeaters.
Where metal walls get in the way I have just used USB extension "cords" and placed the "business end" of Ikea repeaters on exterior walls in a weather-tight electronics boxes (loads of them on Amazon).
I WAS prepared to put one of those on a small solar panel and battery midway IF I had to.
I'll add, in a different location, I have an outbuilding where I have a LAN connection established and a second HE. While it has operated flawlessly I wish it wasn't in that environment and I wish I could have just extended the ZigBee network from one HE (and kept the second HE on the shelf as a backup). That said, there are upsides to this segmentation/separation of roles.
Thanks everyone. A quick scan shows that I am probably barking up the wrong tree with the Hue hub as it is not a generic Zigbee hub and there aren't any obvious contact sensors.
So I think I am going to try the repeater route first.
If the repeater doesn't work, and I know you stated Zigbee but you could also use Hubduino with various temp, contact, motion sensors running on a nodemcu, wemos d1 or similar which connects to your wifi.
You could also just add another Hubitat Hub in the shed, and then use Hub Mesh to tie everything together. It might not be the cheapest solution, but it does offer a ton of flexibility.
I had a similar situation as the OP. So I got one mains powered light switch and wired an external antenna to it to give it some additional range (along with the antenna mod of the Hubitat itself). Everything in the shed now repeats off of that switch. It works well. The shed is about 80 feet from the house.
Download the Hubitat app
