This is just another example of people making up the rules as they go along.
There is nothing in the legislation that states that an exemption letter needs to use any particular phrasing. In fact there is no mention there of any exemption at all.
https://www.gov.uk/government/uploa...324-feedin-tariff--licence-modifications-.pdf
The way this has been worded means that all you have to prove is that neither the middle nor the lower rate applies to an installation. Then you automatically qualify for the highest rate - para 14.
Paras 6 and 7 state the conditions for a lower rate to apply. "relevant building" is just a defined phrase within that.
I can understand why using "relevant building" in an exemption letter is a good idea but I cannot see that it is in any way an absolute requirement.
The issue is also covered from 2.88 in the OFGEM FiT Guidance -
http://www.ofgem.gov.uk/Sustainability/Environment/fits/Documents1/FIT Generator Guidance.pdf
Nowhere does it say that the words "relevant building" must be used in an exemption letter. At 2.90 it does say:
"If an EPC cannot be issued then the building is not a relevant building and the energy efficiency requirement does not apply."
So the mere fact that an EPC cannot be issued means a building is not a "relevant building".
I recollect asking OFGEM at the time that these changes came in what type of letter or certificate would be required to prove an exemption for the need for an EPC and got no reply.
So write back to them pointing out exactly what 2.90 in that document says. All you have to provide evidence for is to show that an EPC cannot be issued.