In the US, an address can, in general, be almost anything that the land-owner intends. In areas with a municipal government (such as NYC) addressing may need to be approved by the municipal planning authority, but this isn't always the case. The USPS will often not deliver to an address until the local postmaster has entered it into the postal service database, but in some cases, esp. in urban areas, they'll deliver mail without this if the address is similar enough to a nearby property that it gets to the right letter carrier route (e.g. a 1/2 or B unit if USPS knows about the base address).

The point of this is that addresses like 1/2, and even 3/4 and so on, do occur in the US, although they're uncommon compared to alphanumeric addresses. If you're in the position of dividing a housing unit it's a better idea to use a B suffix, I used to deal with a 1/2 address and it was constantly rejected by online forms that didn't allow a / in that field.

The USPS has long angled to reduce these problems by assigning a unique number to every delivery point (the ZIP + 4 + DP or '11 digit ZIP'), but this is complicated for all of the reasons you would imagine. Fundamentally the job of interpreting an address still comes down to the common sense of the letter carrier.

USPS P28 has extensive information on how USPS recommends that software handle address formats for e.g. bulk mailing, which is rather lengthy in part because of the huge variety of address variations present in the US.

At this point, might as well just use GPS with altitude. LatLong down to 7 decimal places gets you down to a centimeter or half inch at the equator, better at higher or lower latitutes. Add sea level to the same precision, and you can specify any cubic cm on the planet with about 27 bytes (10 each lat/lng, 7 for altitude offset from sea level at centimeters, giving you 10km of range). Specify your post office box door keyhole, or under your doormat, or in the bush by your porch... it took about as much data to describe these locations in general as to describe the exact location of one

In a country where addresses have to be ordered, if not enough numerical space has been left between existing structures you have to do something, either renumbering (painful and rare) or increasing resolution.

Sherlock Holmes' "222B Baker Street" is a famous example.

In France if you put a building between existing addresses 11 and 13 you might do "11 bis" (or if that's taken even "11 ter"). "bis" might be translated as "again", though it's linguistically related to 2, as can be seen by "ter" which clearly looks like 3.

IIRC (it's been many years) you see this notation in the tree structure of 4G/LTE wireless deployment; the secondary (subsidiary) transmission nodes are referred to as "bis" nodes (I only vaguely remember this because an American engineer giving an otherwise excellent lecture on 4G topology mocked this French terminology).

Bizarre story: In 1984/85 I lived at 11 bis Rue XXX in Paris. Literally the week before last -- 35 years later -- I met an American woman at a dinner in Palo Alto who turns out to have lived at number 13. What a small world!

Seems like it would much more common where the numbering system alternates monotonically for the length of the street. In eg. USA where the numbers go up by 100 per block, there are gaps to make use of.

In a big, old city like Paris or even New York there’s been enough infill over the years that any gals have been consumed.

It would be nice to know what assumptions can be made, instead of having an ever increasing list of ones that cannot be made.

None. The point of "falsehoods programmers believe about..." lists is that "validation" of user attributes is an arrogant and inherently foolish concept, and you shouldn't try.

In the US, USPS Publication 28 specifies the requirements for an address to be considered valid by USPS. The print version is 220 pages and still allows a dizzying variety of formats. In general only the last line (city state ZIP) is standardized and the format of the rest of the address can take many forms. This makes sense when you consider that the last line is the only part of the address used for most of the routing process, the recipient and delivery address lines are mostly only to be interpreted by the actual letter carrier. However, Pub 28 attempts to standardize the other lines as much as possible in order to facilitate computerized validation of addresses (a requirement to receive bulk mailing rates). Addresses which can't be interpreted may go to the USPS mail recovery center (formerly called the dead letter office, a term that stuck around in Linux mail daemons) for investigation depending on the service.

https://pe.usps.com/text/pub28/welcome.htm

And then consider that, on top of all this, while the USPS doesn't like it, just writing the name of a major institution or company on an envelope will still often result in delivery - if it gets to the post office they'll do their best.

Just treat every address as an opaque string. I.E. let the user enter whatever they want.

Both house numbers and street numbers can be a fraction. Washington, DC has both.

In many countries it's common for an entire apartment building to have a single number (or one number per floor) and then a letter for the individual apartments (1A, 1B, 5C, etc).

There may also be an addition of multiple letters. For example: in the Netherlands it is not uncommon to see a number like '45 bis' or even '45 bis B'.

'bis' is latin for 'twice', indicating that the building was split into 2 separate houses at some point in time.

My parents live on a subdivided lot and the letter at the end is used as a decimal component of the house number:

Neighbors up the road have 1234A and my parents have 1234B.

When I moved to Germany I was staying in an AirBNB. The instructions were to just "Ring [Name on doorbell]". When I asked what the apartment or flat number was, it was an alien concept---the host just gave me the address again.

That article actually links to a HN discussion (about the same article) from 2013[0] with more examples, which does indeed point out that street numbers do not need to be integers

[0]: https://news.ycombinator.com/item?id=5791489

Nor greater than zero.

https://rew-online.com/paramount-sells-zero-bond-street-for-...

Wow, there are more: https://www.reddit.com/r/mildlyinteresting/comments/6tk9q4/t...

I can think of one particular, famous example I have been to:

Regina Pizzeria at 11½ Thacher St in Boston's North End

https://en.wikipedia.org/wiki/Regina_Pizzeria

I imagine there are more in that neighbourhood.

I wonder how many of those there are in the US.

I used to work on postal address recognition software and I don't recall that we had any capability for recognizing strings of digits that contained fractions.

It was a long time ago now, but it seems like something I would have remembered.

I used to live at a fractional address and it was pretty much impossible to represent your address accurately in a lot of systems.

I ended up usually writing it as "200h" instead of "200 1/2".

Worked surprisingly well. One of the advantages of having deliveries still carried out by actual people where the address apparently doesn't have to be an exact match.

Yes, I suppose as long as the street name is recognized it will usually go to the right carrier, at least for fairly short streets.

They're not uncommon at all. Surprised to see people in the comments here surprised to learn about them. Might be uncommon in other countries but it's pretty normal in the US.

Not only that. New York has 6-1/2 avenue (https://en.wikipedia.org/wiki/6%C2%BD_Avenue)

I remember seeing some in Poland a bit outside of Warsaw, presumably where a larger piece of land had been sub-divided. I wonder if they fixed that by re-numbering in the last few decades or not.

Yes, if a single fam home is split into a duplex it can end up with fractional addresses if no slack was left in the street numbering scheme.

Sometimes they use A,B,C,D as well rather than fractions.

I had friends that had a 1/2 address in Chicago, sending them mail was a nuisance. Even the Post Office didn't know how to find it some of the times.

In the Chicago area, it's not uncommon for a platted lot to be split in half so that the house numbers would be, e.g., 4012, 4012½, 4014

It's already on there: "Street numbers (and building numbers) don't contain fractions".

A house number is a type of building number.