The reason for refusal here is The extension, by reason of its siting, form, scale and appearance will appear as an overtly domestic and incongruous addition, that will detract from the linear character and appearance of this former agricultural building. See location here.
Derbyshire Dales is close to the Peak National Park and has comprehensive design guidance on barn conversions, upon which it judges any applications for alterations and extensions to barns, and the linear form of the original barns is treated as significant. Although this application is for a very simple brick built addition, which you might say is in the style of the original barns, it breaks the linear form and is therefore unacceptable. It is interesting to see that this old farmhouse and converted barns are completely surrounded now by new build-housing as it it situated in Ashbourne, and so the rural character of the original group of buildings is now lost.
PAPA Says - The changes to the surrounding land could be the argument for an appeal, but the inspector might well still abide by the refusal. Barn conversions do have their own rules which are difficult to overcome.