@Neil.uk-_121 

I would firstly confirm your house extension (a garage/ barn/shed etc.) and the main building is under the same electrical meter.  If they are under the same power meter, powerline adapters should work in the house extension. Considering powerline signal strength loses over distance and most house extension has a certain range from the main building, it is hard to guarantee the performance when powerline devices work in your house extension You could give powerline devices a try,  if that does not work, I would recommend a wired connection if possible for more stable connection.