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The UK system is that the pair ("number", postcode) must be unique for any postal address; "number" is often a house number but things like "1A" or even names l

The UK system is that the pair ("number", postcode) must be unique for any postal address; "number" is often a house number but things like "1A" or even names like "Whitehall" are allowed too. Further, as mentioned, there is a limited amount of "number" that can be associated with any one postcode (currently 100 for new codes, but some legacy postcodes may have more), so for a long enough street, the postcode will change at some point - for example Chepstow Road, Newport changes from NP182LU to NP182LX at some point. If you have more addresses in a single building than the 100 limit, then the postcode indeed changes within the building. This is quite useful as the standard way of entering a shipping address is you type your postcode, and then select the exact address from a dropdown, and there's a natural limit on how long you have to scroll to find yours.

tazjin 3y ago
So a thing that increments "number" is probably also "flat", which probably leads to what I understood as "household" in a large building.
- red_admiral 3y ago
  "Flat" increments "number" if and only if it has a separate post box. If you have several flats behind the same box in a common front door - a common set-up in the UK when a larger house has been converted into several flats - then as far as the postcode system is concerned, those flats don't exist; since people still write "Flat 3, 11 Wisteria Drive ..." on letters this creates various issues with denormalised addresses.
  - tazjin 3y ago
    How much influence does a building owner/developer have over this assignment? Can they explicitly request multiple post codes for a building?
  - dx034 3y ago
    Not always, I've had that exact situation and the flat number was in all systems. All houses in my postcode had several entries for flats and as far as I can tell, they all only had one box.
sanderjd 3y ago
Sounds like a great system! US "zip codes" are pretty useless by comparison.
- Symbiote 3y ago
  As an example, with a postcode "N1C 4DN" we soon learn that means North London (the N), the innermost district (the 1) and the innermost bit of that (the C). Stick it in https://www.royalmail.com/find-a-postcode https://www.royalmail.com/find-a-postcode and we have 5 addresses to choose from. There are usually 10 to 30 — if you work in a large office it probably has a postcode just for that office, for houses you share with 20-30 or so others. (Very large businesses might have separate postcodes for individual departments, e.g. an electric utility probably has one for handling bills, and another for everything else.) "What's the postcode please?" "N1C 4DN" "And the number?" "12" Now they have the whole address. Satnav can take "N1C 4DN" and be very close: https://goo.gl/maps/sGR5XXhmUsLmD2UBA https://goo.gl/maps/sGR5XXhmUsLmD2UBA (not the best polygon, should be Handyside Street.)
  - tome 3y ago
    > with a postcode "N1C 4DN" we soon learn that means North London (the N), the innermost district (the 1) N1 is innermost, yes, but N2 onwards are sorted alphabetically, and N1C isn't really more "inner" than N1. https://en.wikipedia.org/wiki/London_postal_district https://en.wikipedia.org/wiki/London_postal_district
  - stavros 3y ago
    > we have 5 addresses to choose from (8 if you scroll down)
- bertil 3y ago
  I’ve had to deal with postcodes in too many countries (logistic company), and the UK system is by far the best: dense, standard, somewhat intuitive, code-correcting, specific enough (several dozen households) that if you have a delivery, the recipient knows where the van is standing angrily. Documentation is excellent (relative to the UK government's digital service already very high standards), and you have APIs for all sorts of relevant conversions. The only issues are what OC mentioned: some people don’t know a large building (50+ flats) can have several codes, and they are weekly updates because… ::magic dust:: construction! The worst? Dubai: three inconsistent systems of varying length without any sense, standard, or redeeming features. The city road network is apparently even worse, so I guess those things work hand in hand? The funniest? One person once joked that people in Ireland were not using postcodes, just the name of the nearby pub, which can get confusing as they often have the same name, so you also have to say the name of the second nearest pub… I thought was funny, but I wasn’t sure that was a joke. Apparently, that was still true at the time? I saw a lot of discussion about “Introducing PostCodes in Ireland” and avoided those meetings so as not to sound clueless. We used Google Maps for a while during the transition.

tazjin 3y ago

So a thing that increments "number" is probably also "flat", which probably leads to what I understood as "household" in a large building.

red_admiral 3y ago
"Flat" increments "number" if and only if it has a separate post box. If you have several flats behind the same box in a common front door - a common set-up in the UK when a larger house has been converted into several flats - then as far as the postcode system is concerned, those flats don't exist; since people still write "Flat 3, 11 Wisteria Drive ..." on letters this creates various issues with denormalised addresses.
- tazjin 3y ago
  How much influence does a building owner/developer have over this assignment? Can they explicitly request multiple post codes for a building?
- dx034 3y ago
  Not always, I've had that exact situation and the flat number was in all systems. All houses in my postcode had several entries for flats and as far as I can tell, they all only had one box.

sanderjd 3y ago

Sounds like a great system! US "zip codes" are pretty useless by comparison.

Symbiote 3y ago
As an example, with a postcode "N1C 4DN" we soon learn that means North London (the N), the innermost district (the 1) and the innermost bit of that (the C). Stick it in https://www.royalmail.com/find-a-postcode https://www.royalmail.com/find-a-postcode and we have 5 addresses to choose from. There are usually 10 to 30 — if you work in a large office it probably has a postcode just for that office, for houses you share with 20-30 or so others. (Very large businesses might have separate postcodes for individual departments, e.g. an electric utility probably has one for handling bills, and another for everything else.) "What's the postcode please?" "N1C 4DN" "And the number?" "12" Now they have the whole address. Satnav can take "N1C 4DN" and be very close: https://goo.gl/maps/sGR5XXhmUsLmD2UBA https://goo.gl/maps/sGR5XXhmUsLmD2UBA (not the best polygon, should be Handyside Street.)
- tome 3y ago
  > with a postcode "N1C 4DN" we soon learn that means North London (the N), the innermost district (the 1) N1 is innermost, yes, but N2 onwards are sorted alphabetically, and N1C isn't really more "inner" than N1. https://en.wikipedia.org/wiki/London_postal_district https://en.wikipedia.org/wiki/London_postal_district
- stavros 3y ago
  > we have 5 addresses to choose from (8 if you scroll down)
bertil 3y ago
I’ve had to deal with postcodes in too many countries (logistic company), and the UK system is by far the best: dense, standard, somewhat intuitive, code-correcting, specific enough (several dozen households) that if you have a delivery, the recipient knows where the van is standing angrily. Documentation is excellent (relative to the UK government's digital service already very high standards), and you have APIs for all sorts of relevant conversions. The only issues are what OC mentioned: some people don’t know a large building (50+ flats) can have several codes, and they are weekly updates because… ::magic dust:: construction! The worst? Dubai: three inconsistent systems of varying length without any sense, standard, or redeeming features. The city road network is apparently even worse, so I guess those things work hand in hand? The funniest? One person once joked that people in Ireland were not using postcodes, just the name of the nearby pub, which can get confusing as they often have the same name, so you also have to say the name of the second nearest pub… I thought was funny, but I wasn’t sure that was a joke. Apparently, that was still true at the time? I saw a lot of discussion about “Introducing PostCodes in Ireland” and avoided those meetings so as not to sound clueless. We used Google Maps for a while during the transition.