Our old neighbours did this, it was awful.
He was really pushy. Wanted to run a cable under our garden so he could put in electricity and lights. We said no, but he managed it anyway.
It was miserable. They cut down two huge beautiful, mature walnut trees to make space, then built decking over the stumps.
This ran across the back of our house, so All our back ground floor windows became completely overlooked.
One night they had a huge party, and it was so awkward, we felt like we had to hide in our own home.
I wish I'd known about the planning rules, it was so detrimental to our lives.
As another OP has said, they soon realised that we could overlook them as well as vice versa, there was no privacy for any of us. The novelty wore off after about 2 months and they stopped using it.