If the proposed extension is the cross hatched area outlined in red, it looks as though it would be encroaching right on to the boundary which seems quite near the front of your house. Planners like to see minimum distances between houses - eg front to front ,back to back about 21m, gable to front ?
You might have a valid complaint that the proposed extension would simply be too close to the front of your house.
See if you can find if your local planning dept has a design guide for developments in your area.
As others have said, the planning dept could impose a condition on a consent that first floor windows of the extension should be obscure glazed ,also being on the boundary they might have to be fire resisting and fixed shut under building regs ,which could make them impractical for your neighbour's internal layout.
What is the distance from the front of your house to the boundary, about 7 or 8 metres ?
The extension looks too near to the front of your house if built on to the boundary and you could object on the grounds of loss of outlook and possibly overshadowing from a 2 storey extension.
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