I've lived in a one bed apartment where the front and back doors had different postcodes.
IIRC, the neighbours to one side in the same building had a third postcode for their front, but shared mine for the back.
If that weren't true, you'd have entire cities in the same postcode. There has to be a boundary somewhere.
Odd numbered homes on one side of the street and evens on the other often have different postcodes
Postcodes are about sorting mail to match the delivery rounds.
Many buildings also have their own postcode!
(The second half of the postcode represents the 'delivery point' which is basically limited by the amount of post that the postman/woman can physically carry...)
Well, in Italy postcodes define city areas, and cities, for example for my city the main postcode is 80100, but my area is 80142, and it contains few buildings, so it's different from UK, UK was the first time I saw such specific postcodes, and I've lived also in Germany and Netherlands
Netherlands had a postcode per street
Oh yeah, I remember being able to insert just postcode and street number in forms, but it's not as specific as UK, I think