In the U.S. people tend to say housemate for both a house and an apartment—the reason being, no doubt, that we’re less likely to use the simpler word “flat.” Roommates can also be used, even for people who do not literally share the same room.

Well, in the U.S. “roommate” just means someone who lives with you (and it generally implies that you’re sharing the rent).

I say roommate in every situation, even when the two parties live in the same house or apartment but don’t share a room.

And I agree with Jim m., Americans don’t typically use the word “flat” (I don’t even really know what a flat is? if it’s different from an apartment or a house?) but sometimes people live in lofts.

Dutch is consistent with this: huisgenoot (house mate), flatgenoot (flat mate), and kamergenoot (room mate). It also implies sharing the rent indeed, but never used for people who are in a relationship together. The latter tend to share the costs of living as well, so are there any words for people sharing a home without sharing the rent, except your children?

The ‘genoot’ part is something very old. It is derived from ‘genieten’ (to enjoy) in the sense it’s a shared enjoying of something. And not just in the positive sense, because there is celgenoot (cell mate) as well. Oh, cell mate is correct English I suppose. I doubt it’s derived from mating…

I think ‘bydlící’ is a whole word in itself meaning ‘resident’ rather than two separate words byd+lící but I have only a basic knowledge of Czech.
Regarding a word for someone you share a house with but not the rent you’re having a relationship with, maybe a term for this is ‘live-in lover’ or ‘kept man/woman’?

Daviddave – a flat is the same as an apartment – a self-contained housing unit that occupies only part of a building.

Yenlit – ‘bydlící’ is indeed a separate word and means occupant or resident

Spolubydlící is he who lives with you in the same house/flat.

I live in the US and I almost never hear “house mate”. People (at least where I live and everywhere in the country that I’ve been) tend to use “room mate” whether it’s a room, the whole house or an apartment.

IME roommate is the usual US term whether the people are sharing a room, apartment or house. I’ve also heard housemate but not as often.

It may be that the bydl- sequence eventually derives from a stem [by-] ‘be’ plus an affix [-dl(o)] that has a general meaning of something that facilitates the action or process described by the root. It seems to be fairly productive in Czech, so much that I amused my hosts when I was there then years ago by making up nonexistent -dlo words that they nevertheless understood.

Some examples: sedádlo ‘seat’, dívadlo ‘theatre’ from dívat ‘look at’, mydlo ‘soap’ from mýt ‘wash’. There’s also zrcadlo ‘mirror’, though I haven’t been able to find any verb root with the shape zrc-.

Although bydlo apparently has the meaning ‘livelihood’ in modern Czech, and though I don’t have access to a Czech etymological dictionary, It’s probably a safe bet that bydlít derives from an archaic sense something like ‘being-place’.

Jurčík, co myslíte? – what do you think?

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As for roommate versus housemate, I know that roommate is the usual, blanket term in North America, but as for myself (and others in the few such situations I’ve been involved in), sometimes it can lead to confusion and people will say ‘housemate’ instead to make sure there is no ambiguity about the living situation.

Indeed. Verb bydlet (to live) is derived from a noun bydlo (originally living place, today archaic and rarely used), which itself is made of a root by- (present in verbs být, bývat = to be) and the suffix -dlo whose meaning was described in the previous comment. (Zrcadlo is from old Slavic zrkati, not existing in modern Czech, however related zrak (sight) and zřít (to see; slightly archaic) are still used.)

The rest is the transgressive (or gerund) suffix -íc plus adjectival suffix -í, today probably perceived as a single suffix -ící due to rarity of transgressives. Examples of the suffix: sedící pes = a sitting dog, hořící keř = a burning bush, spolubydlící muži = together living men. Spolubydlící is originally an adjective which is now used as a noun.

Cyd-letywr would probably be the Welsh term.

Cyd: ‘co-’ (cydweithiwr – coworker; cydwladwr – co-patriot)

Llety: ‘accomodation’, I guess related to the various Latin/Romance languages word for ‘bed’; lit, letto etc. Lletywr would be ‘accommondatonaer’ or dweler.

Another would would be cyd-breswylwr from from preswyl, but that give the impression of a person staying in a hall of residence (in a college etc.).

These words mutate after the suffix ‘cyd’.

Prase – I did have a quick look to see if I could spot any ‘zrc-’ Czech verbs when Christopher mention he wasn’t aware of any but all I could find were words which seemed to my eyes to be derivatives of ‘zrcadlo’ ie. zrcadlit and zrcadlící (mirror-like, specular, reflective.) with the same adjectival suffix -ící you mentioned.
Macsen – there’s also the verb ‘cydletya’ (to live/lodge together) as well as many other ‘cyd-’ examples:
cyd-forwr – ship/crew-mate
cydanedig – littermate
cydymaith – (travelling) companion
cyd-fyw – cohabit
cyd-dynnu/cydchwarae – teamwork
…to name but a few.

Yenlit – yes, as far as I know the only Czech words including the zrc- cluster are “zrcadlo” and its derivatives. Looking for other relatives one must allow for vowel and consonant alterations. In Russian the original -k- was preserved (зеркало).