When was the garage erected? Did the neighbour get planning permission?
Where was the hedge?
Was it on the boundary?
Are you able to put up a fence on the boundary?
A garage window does not have rights of light in the way that a house window does.
In planning permissions certain windows close to a boundary will be specified to be opaque. You can check whether the neighbour had planning permission by contacting the local council's planning department or by doing an online search on the planning section of the council's website. There may be a condition attached specifying the window to be opaque / non-opening.
Whether you can do anything about it is another matter. There are limitation periods for breach of a planning condition, 10 years from date of breach. So 10 years from construction if installed in breach when constructed or the date it was changed to w window breaching the condition.
If the garage required planning permission and it was not obtained then the time limit is 4 years.
If you can, you may want to erect a 6ft high boundary fence to regain privacy.
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