Scenarios: What do I need for...
... a farm?
Working—The farm buildings will all require commercial EPCs and any residential units on the farm will require residential EPCs.
Non-working—It will require a HIP unless it fits one of the exceptions.
... a house with an additional dwelling ?
A HIP is required for the main dwelling and all additional properties. Flats and annexes will require separate EPCs.
... a barn being sold with planning permission for a dwelling?
If the barn will be in the same, undeveloped state, at completion and does not have any fixed heating installed it is exempt from needing either a HIP or an EPC. If there is heating it will need a commercial EPC.
If the barn will be a dwelling at completion of the sale a residential HIP will be required, as long as the dwelling does not fit one of the other HIP exceptions.
... a property let as a holiday cottage?
If the property can only be used for holiday rentals, as defined in the planning permission then it will not require a HIP, only an residential EPC. If the property is used for holiday lets but not restricted to this use then it will require a HIP, unless of course it fits one of the other exemptions.
... a shop and flat in one building?
The shop will require a commercial EPC and the flat a residential EPC.
... equestrian facilities?
All commercial buildings will require commercial EPCs and dwellings will require residential EPCs.