@Pk2 

I am assuming right now you have no Internet/Ethernet connectivity in the garden office, correct?

I take it that you've already tested WiFi connectivity between garden office and main house, by placing Satellite X20 in garden office and checking if it can reach any X20 from main house. If not, you should do that.

If garden office is fed with electrical power from the house, you might be able to use powerline adapters to deliver Ethernet link to the garden office and connect Deco unit through that link. 

If it is, let me know and I'll provide more details.

If by any chance you have coaxial (TV) cable going from main house to garden office, that could be another option. I am guessing this is unlikely case, but mentioning it just to cover everything.

  @Alexandre. 

Thanks for your detailed reply. 

Yes i have tested my current deco x20 and it does not reach the garden room. 

We do have electricity in there so best to use powerlink adaptor I Guess. Do I need to buy 2 ?

Thanks

  @Pk2 

You will need to buy pair of powerline adapters. If you want to extend Deco mesh, those should be compatible with Deco Ethernet backhaul. Powerline adapters from TP-Link brand are safe bet, for example "TL-PA7017 KIT" or similar set with pass-through socket "TL-PA7017p KIT."

Check that powerline adapters can establish connection between them, from garden office to the house. If they can't, this may be because electric wiring having issues or they are not on the same circuit breaker. 

When you do have powerline adapters connect to each other, bring one of Satellite X20 you already have to garden office, connect that X20 with Ethernet cable to powerline adapter Ethernet port and check X20 connection to Deco mesh. If everything done right, you should see X20 reporting Ethernet backhaul connection in Deco app. 

Then, if everything works, return that Satellite X20 back to its place and buy additional single X20 or X50/X55 specifically for garden house.