Agriculture is generally exempt, but Building Regulations will apply to any structures likely to be inhabited by people for any length of time eg farm cottages and commercial premises. The Local Authority should be consulted about the application of Building Regulations to a construction project before work commences.
If applicable, the project should be registered and a building inspector instructed. This does not have to be a Council inspector and it is often cheaper to instruct a private contractor as long as they are certified to sign-off the building work.
The inspector can enforce changes to ventilation, lighting, drainage, fire prevention, etc if they do not meet minimum standards. It is cheaper to get these things built correctly from the start than have to change them later!