Small home, no storage
If you live in a flat with nowhere to put anything
The most common reason people give up on this entirely. It is solvable.
The honest version
You need about the space of two shoeboxes, not a cupboard.
Water goes under the bed
Nine two litre bottles lie flat under a bed and are invisible. That is three days of drinking water for two people.
Food is not extra food
It is the food you already buy, one shelf deep instead of one day deep. Buy the pasta you were going to buy anyway, one packet earlier.
Height is the real risk in a tall building
If the power goes, the lifts go, and in many buildings the water pumps go with them. Fill containers in the first ten minutes and know how many flights you are from the ground.
Find out who is responsible for the building
Managing agent, freeholder, housing association or landlord. Get the out of hours number now and keep it on paper. On the night, nobody can find it.