The law related to how your building works may affect the light enjoyed by your neighbour ( assuming your neighbour has acquired a right to light ). You may not build, or cause to be built, any works on your own property which common sense suggests may cause a reduction in the normal light requirements or inconvenience to your neighbour's dwelling. This is true, whether or not you have the appropriate planning permission and/or consent under building regulations for your own works on your own property.
Such legal generalities inevitably lead to potential legal interpretation and argument if your neighbour considers you are placing him at an unfair disadvantage. The amount of normal light requirements won or lost by new building works is not an easy matter to evaluate; but the plaintiff must prove substantial they will cause, or have caused, an interference to the normal enjoyment of his premises which would constitute a 'nuisance'. You should avoid engaging in a 'neighbour's from hell' dispute which can only end in tears. There are a number of guidelines that must be considered to ensure your neighbour's rights are upheld by whatever you intend to build for your own enjoyment and profit. These should include but not be limited to the following:
Commonsense must suggest that any perceived reduction in normal light requirements you might cause to your neighbour's house cannot be construed as causing unreasonable discomfort. It is advisable to obtain your neighbour's permission in writing before the works commence (with or without compensation for loss of value of his property)
If your works are major and require some easement or other legal requirement before your works can commence, then it may be necessary to engage a solicitor to insert an appropriate permission clause into your's and/or your neighbour's Property Deeds
The light to the existing window(s) of your neighbour's house deemed to be affected by your building works must have enjoyed complete freedom from any blockage by whatever cause for at least twenty (20) years, unless the property has been unoccupied for that period of time
Your neighbour may have pre-emptied your actions to build works that could invade his privacy and already secured legal protection against his neighbours obtaining a right of light over his land.
If you build works on your own property that subsequently can be proven to have unfairly affected the normal light requirements to your neighbour's window(s), then your neighbour has the right in law to persuade the Authority to place a Demolition Order on your new works. Any such demolition works upheld in Court would be all at your cost
A new window built on your property should not cause unfair visual access to your neighbour's property without obtaining his agreement in writing (with or without compensation for loss of value of his property).
Light is free to all with no special rights for any one person; on the other hand there must not be any infringement such as could cause a nuisance.