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 residential EPC.

... a house with an additional dwelling ?

An EPC 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 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 EPC will be required.

... 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.