This is mainly for @Crofter, depending on how Wee the new Hoose actually is - put here in case anyone else finds it.
In England, detached buildings under 50sqm are exempt from having an EPC done.
https://www.gov.uk/energy-performance-certificate-commercial-property/exemptions
If it is also so in Scotland, that could save you a little money and hassle - if you are under 50sqm and so minded.
I have one under 50sqm - the detached 48sqm cottage my dad was born in, which is now a rental. I have the EPC number up to 51, and the half of it which has been drylined since about 1984 (25mm eps) was ignored by the EPC man, but that is about the limit of what is achievable practically. When push comes to shove in 2026 I will rely on the exemption rather than make a "too expensive to upgrade it" argument.
Ferdinand