Taking Boris Johnson's 'garden party' easement way too far!

I need some advice. In the last week, my neighbours erected a large detached annexe without so much as speaking to me before they did so. They have no planning permission either. I have lived in my home for 35 years and they are the same neighbours who have been there for 36 years. They know all about my son because the woman was a good friend for 15 years and used to come into my house more or less daily. Then her daughter started bullying my daughter and hit her, by punching my daughter in the head (my daughter was younger and half the other girls size at the time) Anyway the friendship ended over this.
Since then it’s been hello/ goodbye and the very occasional chat over the fence maybe once every few years.
Then suddenly, they say they are building a bench…and an imposing annexe is erected complete with foundations and electrics at the end of their garden facing my house directly. In an elevated position so high it faces my upstairs bedrooms, bathroom as well as looking over my entire garden and downstairs rooms…invading my privacy. They may as well be in my garden…The daughter’s child who lives there has been running all over the top of this shack.
What’s worrying me most is the noise when my son is trying to sleep.
Since they’ve built this they are avoiding speaking, by turning their backs on me, when they leave their house. I’m sure they know full well I wouldn’t be happy about this building they have just put up.
I just don’t know what to do and I’m really worried about my son as he’s noise-sensitive and it causes seizures.
Any advice would be appreciated.

Why is it elevated? - Is it on a hill?
One-storey structures (garden studios) don’t need planning permission if they keep within certain limits. If it’s got more than one storey it’s a different matter.

Ring the planning enforcement officer at the council and let them deal with it.

Hiya Greta, I know they can build within planning permission regs such as a metre away from perimeters and a certain length and height. It’s elevated to an unreasonable level and the picture was taken upstairs to show how high it is. They have a small garden inner terraced, the back of it is not far away from the back of my house. The fact it’s a permanent build complete with electric sockets means it’s a bit more than just a garden studio although I wouldn’t call it that. They plan to put another window on the other side where it’s open at the moment.
I would have had no problem with it if they had dug down before making it a permanent structure. It’s the fact it overlooks my home so badly and they are known to sit in their garden at all hours talking loudly/laughing and not caring less if they wake my son who will scream the house down for hours after. When my son gets anxious it brings on seizures. He could die.

Hiya Bowlingbun, I’ve sent a complaint to the planning department via email.
It that bad right now I’m actually thinking if the planning department allow it, despite its breaches. I will then be submitting plans for a 3-bed bungalow, on the side garden, which I will move into with my son…and rent out the original house to someone else.

Bowlingbun’s advice is right - the council can deal. I called it a garden studio because that’s one of the terms used, e.g. by estate agents. You could say outbuilding or something else. Apparently they can be up to 4 metres high. It’s normal for them to have power and an internet connection. I can understand how you feel and let’s hope you can get it pulled down. But if it’s within the law, you’ver got a problem.

Hiya Greta, Yes bowlingbun is always spot on with her advice. I know the Council is going to be slow about dealing with it and renowned for turning a blind eye, I’m sure. The worst thing is the neighbours have just shoved it up and it looks like a cross between a watchtower, a storage container turned on its side, and a ninky-nonk.