What size extension can I build without planning permission in 2024?
However, any rear extension classed as large (longer than 4m for detached or 3m for everyone else), needs prior approval which involves consulting your neighbours on the development. There are also height parameters. The extension needs to be less than 4m in height (or less than 3m if within 2m of a property boundary).
and
50% rule: extensions or outbuildings over and above the original house must not exceed 50% of the curtilage (the land around the original house) No higher than the highest part of the roof of the existing dwelling house (eaves or ridge)
however this is usually for residential purposes, as theirs is a business I am not sure how they stand, perhaps best to contact planning to ask the question.