I can't see why you MUST connect a building to an electrical supply .

Obviously some garden sheds , garages , farm barns and the like aren't 'powered' .

Things like rental properties must be 'suitable for occupancy' , so you can't rent an unpowered house as a house , and in some cases you must even supply heating , heat pump , as well as working stove and shower ( I'm paraphrasing obviously ) . It is possible to provide most of that with gas or even a generator , but if it really came down to it , you'd need to disclose this before renting it out .

To to revert back to a 'legal' distinction , these are rental requirements , not building code requirements .

Having said all of that , you would need some sort of 'energy' supply , whether it be electricity , gas , generator , solar , your own dam or windmill , etc , and generally electricty is cheap to install and use , so it makes sense to have it installed or at the very least planned at construction .

Slightly off subject but similar , a friend of mine bought an older caravan and it is way up the back of his farm -- he uses it maybe 1 night a month . He bought 2 large batteries , a 2000 watt invertor , some solar panels , but as a backup he bought a small generator that runs on LPG ( propane ) and as he cant get a vehicle close to the caravan , he carries in a 9kg bottle ( he has 2 so when 1 is empty he carries it out and gets it refilled for the next visit ) . He thinks that is better/easier than petrol or diesel .