In the US is there a standard definition for an apartment, a flat and a condominium?
You can apparently either buy or rent an apartment or flat, however you only buy a condo. I live on the west coast and we normally don’t buy apartments, but in Manhattan apparently they do. 
Condominium carries a specific legal meaning (a horizontally and vertically defined unit of a multifamily building with common walls and facilities, so recorded with the county). Beyond that, it’s entirely local usage, like soda, pop, and soft drink.
Well, actually, I could buy a condo and rent it to you.
The way I think of it is you buy a condo, you rent an apartment, and flat is a regionalism or British term. But the there’s townhomes and the such.
One definition I’ve heard: In condominiums (condominia?) each unit is a fully self-contained living unit, having its own heating, water heater, and other such appliances that might be shared in apartment buildings.
In apartment buildings, at least those I’ve seen, it is common for several units to share a common water heater. I’ve never seen one with shared space heating, although I’ve heard of the style of having a boiler in the basement with radiators in the units that the tenants can control.
I inquired once what was meant by the phrase “condominium conversion”, and that was the answer I got.
In a condominium each unit is individually owned.
In an apartment house, the entire building is generally owned by some entity, which then rents individual units. If you can buy an apartment, then it’s actually a condo.
As has been mentioned, you can rent a condo, but you would usually rent it from the individual owner rather than the building owner.
“Flat” is a British term that is not generally used in the US.
It’s not very common, but there are single-family houses owned as condos.
“Flat” is a britishism meaning “apartment.”
FYI, most purchased (ie, non-rented) apartments in NYC are not condominiums. They are co-ops. This is a crazy form of property ownership in which each person owns a share of the building and the right to live in an apartment. But they are not owners of the apartment itself. They are technically leasees, and they can’t rent or sell the apartment without the approval of the other owners (usually delegated to an Co-op Board, which is kind of like an HOA but with more authority to fuck up your life.). It’s kind of like the worst of all possible worlds, and, AFAIK, only common in NYC and Chicago.
Oh, yes. Co-ops. Forgot about those. They do exist here in Chicago, but they’re not super common to my knowledge. I hadn’t even heard of them until about a decade ago when I was first looking to buy property (but thankfully waited.) Never quite understood their appeal.
So… you’re essentially leasing an apartment from yourself?
The first apartment I rented in the Denver suburbs after I left school became a condominium. Condo conversions were happening all over then. I basically got a letter saying “Buy your apartment or move out.”.
You’re leasing an apartment from a partnership that includes yourself.
Once I dated a British woman. She criticized my apartment so I knocked her flat.
A friend of a friend owns an expensive CoOp in NYC, and would definitely agree.
There are two different issues- one is the style of home (apartment, rowhouse or townhouse, detached, semi-detached) and the other is the type of ownership. Any of those styles can in theory be rented - I say in theory because co-op associations require that the board must approve buyers, and may not allow renting.* Any of those styles can be owned as a condominium- there are single family detached buildings where the owner owns only the interior of the building. The land and exterior of the building are common property and maintained by the condo association. When someone refers to “buying an apartment” , they are referring to a co-op or condo that is apartment style- one unit over the other, or side by side with a common entrance, or multiple floors with multiple units on each floor.
Condominiums became popular fairly recently. Until their popularity, ownership of an apartment would be in a percentage of the co-op, a cooperative association of all the owners of the building (forming, in effect, a corporation).
Co-ops fell out of favor in most places, even in Chicago (although still used much in NY) because the mortgagees of the owners do not favor co-ops. In order to foreclose a mortgage on a co-op apartment, all the owners of all the apartments are necessary parties because they all own a share of the “corporation” which is the building. In a condominium, the owner has complete title to his unit, which is everything within the exterior walls, and a percentage of all the common elements, To foreclose a condo unit, the owner of the unit and the homeowners’ association are the necessary parties.
As Mr. Downtown points out, initially condominium was for horizontal divisions within a parcel of land, taking advantage that one owns a parcel of land up to the sky. (Initially, airplanes could trespass because of the theory of implied easements, but I think now the ownership goes up as far as reasonably can be used.) However, more recently, the condo ownership was applied to townhouses, as a legal construct. The horizontal division really does not apply to a townhouse condo. In a townhouse association, the owner of a unit has easements over all the common elements. Thus when selling a townhouse, the deed must contain a description of all the easements. When selling a condo unit, the unit together with its corresponding percentage of the common elements, is conveyed.
It’s not a “corporation,” it’s a corporation. In particular, it’s a specific type of non-profit shareholder corporation called (drumroll) a housing cooperative.
Mortgage companies (at least in the east) are still perfectly happy to write loans for co-ops (I’ve had two) but they require a little bit of specialty knowledge which is why banks in areas where co-ops are uncommon generally avoid them.
The co-op corporation confers limited liability on the shareholders, and it is the corporation plus the individual shareholder who would be parties to a foreclosure; other shareholders would not be involved. Indeed, if a neighbor of mine were foreclosed upon, I doubt I would even know about it. But the board members would know.
SF area:
The Apartment/Flat distinction is this:
When you walk out the front door, where are you?
If you are in a hall, it is an apartment.
If you are outside, it is a flat.
Flats are much more expensive than apartments.
Structures built as huge apartment complexes were split into “condos” and sold. This was the 1979 - 1983 era when house prices skyrocketed - and the owners of the apartments saw the money to be made. A $350/month apartment could be sold for $60K. A $500/mo brought $80K.
Many areas enacted laws regulation conversions - they wanted to retain rental housing.
In SF, there was a lottery to get permits to convert. This is when “shared ownership” became popular - a group would pool money and buy a building, holding it as joint ownership (do not remember the legal name). Yes, tenants-in-common (ir whatever) are very difficult to finance - bvery few lenders will loan based on “your right, as granted by the others in the place, to use the second room on the third floor”.
If you are offered a “condo” in Concord CA, look at it - hundreds of tacky, flimsy apartments were converted. If it is anywhere near the intersection of Laguna and Detroit, it was a crappy apartment - no double walls, no individual water heaters. Hear the neighbors breathe apartment.
I almost bought a purpose-built condo in Concord. I tracked their prices. From $85K they got to $300K a the peak of insanity. A house in SF would have gone from 85K to 760K in the same time.
Location, location, location.
I’ve lived in the Bay Area for almost 30 years, and I’ve never heard this.
It’s mostly in San Francisco itself, I don’t know if it extends to any of the suburbs. When the Chronicle still had rental ads, there were separate categories for Apartments, Flats, Condos/Townhouses for rent, and Houses for rent.
Further about flats in San Francisco: in other locales they might be called a vertical duplex or triplex. The upper floors have self-contained inside stairways leading down to an outside door on a shared porch. Each floor is generally one unit, unless they have been further broken up, in which case they are no longer considered flats.