In the main any outbuilding comes with three options.
1) The fused connection unit, so total limited to 13 amp.
2) The MCB or RCBO this varies consumer unit to CU and the maximum size varies from 32 amp to 70 amp. Only seen 32 amp limit in old Wylex and 70 amp with the old load master, but often 45 amp is the largest that will fit.
3) A henley block and basic same limit as main house. Often variation from this with an isolator but not a MCB or RCBO in the main CU.
There is a 4th where the same CU does main house and flat, this is what I have, so total of 14 RCBO's.
Number 3 needs same size cable that feeds the house or more, likely 25 mm² cable, where as number 2 you can get away with smaller cable, and number one will not suit you.
This is old so a little outdated however it goes through the considerations for an outbuilding.
We need to consider what earthing system to use, we need to look at the voltage gradient if not same as main house, and so distance from main house matters, and today the likelihood of charging EV's, and what happens if a RCD trips.
For a living caravan it is considered 32 amp is the smallest supply, just over 7 kW, and some homes only have 60 amp to start with, so you need to look at the existing supply first. With a caravan we have battery powered lighting so there is little danger if a RCD trips, not however with a flat, so likely the supply will come before any main house RCD.
So what you are looking at is design, and it may be using a gas cooker can save a lot of money, so it needs a lot of planning.
