I’ve been through something similar twice before.
First house we bought, the people at the back applied for a 2 storey extension that came right up to our boundary but didn’t include any overlooking windows. We objected to it as it overshadowed our garden. Neighbours either side did exactly the same. It was allowed and once they started building it a window appeared in the first floor. Called the council and explained this to them. The response was “oh no, you can’t be right as thats not on the plans” as I was stood in my kitchen staring at it. Long and short of it was that the planning dept/officer involved was utter rubbish, allowed the development to go ahead with first floor windows looking down into our gardens. The guy next door who was a bit of an odd guy was in the middle of constructing what can only be described as a massive billboard a foot away from his back fence when we moved out.
Lesson learnt? Get in there early, read up on whats allowed and whats not. And keep going. Make a real bloody pest of yourself.
Second time in our next house in a conservation area someone put a dormer on a house opposite. Under the rules of a conservation area stuff like this has to be pre-approved, must be in keeping with the area, mustn’t drastically alter a roofline etc etc etc. I became an expert on conservation area development! Anyway, this dormer was full width of the house, clad in white PVC with PVC windows (rest of the house was original sash windows). I spoke to the council as no planning had even been presented to them so they were interested in it. Again, long story short, my logical and informed argument led the dormer to being drastically reduced in size and made more in keeping with the area.
All these people saying move on etc, oh please fk off. You live there, its your privacy thats potentially being invaded. Do everything you can to stop it if you think its going to ANY effect on your use of the garden etc.