The oddest thing is why someone would want a feature window providing a direct line of sight from several gardens and houses right into your bed. There's not even a view.
And the cost for that extension would be, based on bailing out of something of similar scale about 2 1/2 years ago because the dream wasn't worth the cost, would be about £300k or more. On that basis I suspect the properties in that area are all worth about £500k. But no matter what they have done, it's still a suburban semi detached so they are never going to recoup the investment, particularly as the result is so hideous.