is it simply a summerhouse, or has it got water/electricity? Do you think they are going to AirBnB it, as lots of people do this, under the radar of the council
I live in a national park, but on the edge of a town. The neighbours opposite built one of these in their back garden. It has a shower room and toilet.
They were told that as long as it didn't have cooking facilities, it wouldn't require PP. The letter from the council giving this advice is on their planning portal website.
They rent it out on AirBNB and at the moment it seems to have almost 100% occupancy. It's clear from the listing that there are no cooking facilities, but there is a kettle and a microwave.
It doesn't bother me, as my house is up higher, plus the roof is planted with sedums and stuff so it's not obtrusive, but the people on the far side say it really looms over their garden and has cut out the sun from the bottom of their garden.
All the houses on my side of the road used to have views of the downs through the gaps between the houses opposite. Nearly every house now has a side extension taking up the full width of the plot, so they've lost their views. When people have objected to these being built, they're always told that loss of a view isn't a valid planning objection.