The lawyer Simon Ricketts, Partner at Town Legal LLP, has written an article on this subject, following a clear judgment from The Hon. Mr Justice Holgate of the High Court as to how decision makers should approach applications for prior approval for the upward extension of buildings under the General Permitted Development Order: CAB Housing Limited v Secretary of State (3 February 2022). Civic and amenity societies should consider the conclusions extracted in the article.
Barnet Council has had several successes in refusing upward extensions which they considered would cause harm. One was a dismissed appeal here.