A lot of the Grade 1 listed buildings repurposed into homes have glass and metal additions that clearly delineate between old and new for this reason. They can be incredibly expensive jobs though due to all the permitting and oversight. This looks like someone on a budget or who ran out of money and couldn’t afford a more aesthetically pleasing addition. If it’s being lived in though, they will stop it from deteriorating and it will help to preserve it, which is the only reason it’s allowed in the first place.