You never know who might turn you in or why, so it’s worth checking with your local planning department before starting work, especially if the arrangement is expensive. I know of a plan where a tennis court was built, which extended just beyond the garden landscape but was surrounded by trees. It was reported. How it was seen is a mystery, as it was in a very isolated spot. It got approval afterwards – which was lucky, because the costs of removal for tennis courts can, surprisingly, be higher than the costs of construction.
Mistakes can be made when people think that their entire plot has been designated as a garden and therefore they have granted development rights over the space through which to build certain functions. But sometimes remote areas, such as an old orchard or paddock, can be considered farmland rather than gardens, which are not automatically covered by development rights.
Permitted development rights allow owners to build sheds, outdoor offices, paving and fences, as long as they meet established size limits and you do not live in an Area of Outstanding Natural Beauty, a Conservation Area, a National Park, a World Heritage Site Site, or the Norfolk or Suffolk Broads.
One of my clients bought a field with the intention of expanding their garden with additional lawn, but they were instructed to mow it regularly as this meant it was no longer in agricultural use.
In another case, the owners were not allowed to establish a garden on their adjacent paddock and built a vegetable garden and greenhouse on it, which was deemed acceptable as it qualified as horticultural production.
For monumental buildings, you may have to apply for a monument permit for many elements within the plot. I’ve had to apply to remove a 1960’s prefab greenhouse (a bit of a monstrosity) because it was attached to a protected garden wall.
Ornamental pools, low walls, large areas of pavement, and even raised beds may require permission. I’ve come across instances where planners have had animated discussions among themselves about what permissions are needed, so it’s no wonder Joe Public’s perception can be vague.
If you’ve developed something without permission, you could be forced to remove it unless you’ve had 10 years of continuous use.