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.

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

"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.

How much influence does a building owner/developer have over this assignment? Can they explicitly request multiple post codes for a building?

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.

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

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 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 (not the best polygon, should be Handyside Street.)

> 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

> we have 5 addresses to choose from

(8 if you scroll down)

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.

I mean it's funny, but I don't know if I'd call it true. It was more that if you didn't know an address you could bet that the pub would help direct. (Probably wouldn't work anymore because all the rural pubs are closing)

Theres a difference between a system and embodied knowledge. I did a lot of work with systems which used Irish addresses early in my career.

"An Post", pre-eircode operated off a traditional hierarchical address system. Where there were "Counties" a real political boundary, "Post towns" which were usually big market towns but the location of a major sorting office, "Localities" (sometimes more than one) which were geographically undefined (we tried) at worst areas and some combination of street and buildings. The hierarchy was not strictly defined, it was a bit hungover.

My own address can be a combination of: <House number> <Street>, <Post town> <House number> <Street>, <Locality>+, <Post town>

Most of these were optional. The "Pub" thing is a testament to how awesome the staff at An Post are at just getting a letter to a door. If you needed to send a letter to "Mary O'Shea" who you knew lived in Kerry and near "Paudie O'Sheas pub" you can bet that if you put "Mary O'Shea, Near Paudie O'Shea's Pub, Kerry" you can bet the letter would get to Kerry, someone in Kerry would know Paudie O'Sheas is in Ventry, Send it on to Dingle and he postie doing the rounds in Ventry would be like "Ah right, thats for Mary" and the only catch being: about 60% of the Female population of West Kerry are probably called some combination of "Mary" and "O'Shea".

I'd imagine logistics companies were dancing for joy with every house having a unique post-code.

The US ZIP+4 code (five digit zip code with four digit extension) does this. But nobody uses the ZIP+4 it seems.

That's a bit chicken and egg; I have my ZIP+4 in 1Password (mostly so I don't forget what it is) but that also means it autocompletes the full ZIP+4 into form fields. I'd wager it's easily 90% of the forms that flag the field as "invalid," forcing me to delete the dash and 4 numbers. And I'm not talking about the setups where there is a separate textbox for the +4, I mean the inputs are always "but a zipcode is 5 digits hurr"

So, with users being actively taught not to provide it, of course no (reasonable? :-D) person is going to know it and thus provide it to make it available for use

It is quite valid for one property to have multiple street addresses and therefore postcodes. It could genuinely have frontage in two streets, or it could be the result of joining two properties in the same street that originally had different postcodes – many long streets have multiple postcodes along their length¹.

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[1] For example even a not-very-long street I used to live on, Alma Terrace in York, has three postcodes: YO10 4DJ on one side of most of it, YO10 4DL on the other side of that, and YO10 4DQ for both sides of the part between that and Fulford Road. I suspect from the street layout that the third code is due to that part of the road being added later, or originally not having anything on it needing a postcode.

And it's also valid for UK postcodes to be reused, e.g. following demolition of the original buildings, e.g. tower blocks, the postcode may be deactivated for a period of time, then reactivated when new buildings elsewhere need a new postcode.

In London what ends up happening is that many buildings are so large (e.g. St. George's Wharf[0]) that the household count restriction ends up with multiple codes being allocated.

[0]: https://upload.wikimedia.org/wikipedia/commons/c/c1/St._Geor...

Any info on how could I have my street address corrected on Maps?

They are using a random street name that no one else uses and no matter how many times I report it, they don't change it. It also doesn't matter that I'm a local guide with many, many edits.

If you work there you can maybe reach someone, but at this point the company is so large that even that doesn't help anymore in many cases ;)

You can ask a few friends to report it as well. I suspect Google looks at how many reports are made for a feature.

This is also valid in Japan, there's huge build with a mall, a hotel and residences called sunshine city in Tokyo, every few floors has it's own zip code!

https://www.post.japanpost.jp/cgi-zip/zipcode.php?pref=13&ci...

Search for 170-0013 for the beginning of the madness. At least google accepted this one.

I think this is actually a really perfect demonstration of why you often shouldn't attribute things to malice which can be explained by incompetence.

This is on my mind a lot when I see speculation about why google (or other large organizations) does various things. It's just a bunch of human beings with egos and biases and blind spots and imperfect information. Mistakes are made.

I'll bet he can reverse a binary tree blindfolded though.

But that is malice. They're specifically fucking over an entire country because they believe that post codes shouldn't be that way. They know otherwise.

No, it isn't. It's ignorance. It's incompetent to be ignorant of something that matters to doing a good job. It's possible to remain ignorant about something despite being told the correct information. (This is the difference between "ignorant" and "unaware".)

Malice requires intent to do wrong, either for some selfish benefit or just to be cruel. That's not what's going on in this story, it's "just" ignorance.

Thank you for that insight. That explains a lot of the bizarre design decisions and shortcomings in the maps UI that have frequently annoyed me.