To consider this an eyesore

[OnlyFoolsnMothers]

House behind mine, not directly behind- behind and to the left are building a summer house, however the structure is up and it’s huge! More of a full blown house. I’ve tried googling and annoyingly I’m not sure they would have needed planning permission. Would you hate this although not directly behind my house? ...I’m also dreading how much “hosting” they will do in it.

If the falling down fence isnt yours then does that mean the new structure is two gardens along from you? Unless you go upstairs & hang out the window looking left as you did for the photo I dont think it's going to be in your sight much?

My neighbours either side have summer houses. One is used for band practise and drinking, the other for a gym. Can’t say it ever occurred to me to have an opinion. I do occasionally encourage the band to move on if they’ve been playing the same song for an hour but it part and parcel of living in a city.

I think it's pretty bad OP.

Planning permission applies to buildings without excavated foundations. The height of the building is key and the intended usage.

I live in a terraced house and the only way I can check out my neighbours’ gardens is to stand on the side of the bath and look out of my bathroom window. Apropos nothing at all.

Is it when building services such as electrical, water supply that the planning permission criteria is met then?

I still can’t get over how much this bothers you when you have that ugly fence, priorities? Sort out what you can actually see from sitting in and enjoying your garden rather that what you can see over next door’s garden from upstairs.

From the pic it looks like you ems up seeing less of neighbor's garden, which is a good thing in my opinion.

I'm not an expert in planning firehousedog but I installed a cabin last summer so checked out what was needed. Most cabins will be allowed as permitted development if they meet certain criteria. Outside those criteria planning consent would be needed.
www.iobuild.co.uk/garden-office-planning-permission/
This explains it I think

Get some fast growing plants along your fences if you want some privacy plus it will just look a lot nicer.