Our neighbours decided to quickly erect this prefab on Friday, and as its on a raised garden, its very imposing, totally overlooks our garden and blocks the view. I'm concerned it could effect our house price too. 

I can't accurately figure out the height, but it looks around 3m or so, and is about 3.4m away form our fence(I dropped a laser measure over). And will be under the 30 sqm limit. 

I've spoken with the council and seems there is not much we can do. If they use it as a second dwelling they would need consent, which we suspect they might, as there are a lot of people who seem to live there and come and go, and a huge amount of furniture that was delivered (but too early to tell).  

Our only thoughts are to put up the biggest fence we can, and grow some tall trees.

But I'd love some other insights, thoughts on what we can do.