It’s a planning matter, right? Well, no. Amenity is indeed a planning consideration, but formal 'rights to light' is something different. Put another way, you might convince the planners that your proposed building work is fine, but a neighbour could still have a case against you in court.
Let’s explore why. Essentially, a person’s right to light is a matter of common law and in England and Wales is covered in statute by the Prescription Act 1832. In effect, a window that has received more than 20 years of unobstructed daylight has earned itself ‘a right to light’. So even if you have planning permission, a neighbour’s rights could trump it.
Therefore, it’s important to consider who might be affected, by how much and what they can do about it. Sometimes neighbours have waived their rights (for example, if their property has been built within the grounds of another). Also, some loss of light is acceptable. As a rule of thumb, your neighbour needs to show that the overall reduction makes the property less fit for its purpose. What constitutes a significant loss of light depends on the use of the property and its design and will vary from place to place and from building to building.
So don’t confuse planning permission with rights to light, and if in doubt get some specialist advice.