On the face of it, this is a perfectly acceptable Class Q, agricultural building to house approval. The scheme is very simple, with limited windows inserted into the existing building envelope, and so you would not expect a refusal.
There is however never a straightforward decision, and in this case it comes from the ground upon which the existing building sits. The decision notice reads The site lies within a Minerals Safeguarding Area identified for its underlying limestone resource. Policy M2 of the Devon Minerals Plan 2011 –2031 (adopted 2017) and paragraph 212 of the National Planning Policy Framework seek to protect mineral resources to enable their future extraction. The proposed conversion of this building into residential use would constrain the future extraction of the underlying limestone resource for which this area is safeguarded.
PAPA SAYS - Once again this is one which should go to an appeal inspector to decide. If the building is already there, then future mineral extraction will basically destroy that building. If the applicant/developer is fully aware of these restrictions, it should be their decision as to whether they wish to take the risk of converting to a dwelling, only to possibly be up-rooted in the future .