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Why doesn't

A three room house

have the "ed" ending while

A three-headED dog

has?

It is perfectly normal to say a three-roomed house, a four-engined plane, a three-wheeled car, a five-sided coin. Many such constructions are also valid without the -ed form. So your question "Why doesn't 'a three room house' have the "ed" ending?' is based on a false premise.

In Britain, houses are commonly classified by the number of bedrooms, and you can see sale announcements using either 'X-bedroomed house' and 'X-bedroom house'.

'Eight Roomed House' (2009) was the debut novel of Fiona Margaret Watson.

Often, a phrase like: a house with three rooms, can be made to function like an adjective by writing:

• a three-room house [many sites with leave out the hyphen, but in formal writing, it should be there]. This is standard in the AmE.

Here is a full explanation of this, which is called a noun adjunct, attributive noun or noun pre-modifier: noun adjunct. This can get complicated but the Wikipedia entry is rather thorough.

The second point in the question concerns parts of the body used as adjectives in English. This point is more straightforward. It so happens that traditionally English adds ed to body parts to form adjectives with numbers:

• a two-headed monster [with two heads]


• a three-legged dog [a dog that has lost a leg]


• a one-eyed man [in literature, often a pirate with a black patch over one eye. :)]


• a three-toed sloth [a very cute animal, by the way]


• a six-toed cat [polydactyl, can also have seven, by the way].


• a one-armed man [a man with one arm]


• a fleet-footed animal [a deer, it can run fast, for example]


• Three-eared Rabbit [a restaurant in Trussville, Alabama]


• a dog-eared book [worn or in bad condition at the corners]


• a knock-kneed child [the legs curve inward]


• a scatter-brained person [one who gets confused easily]


• harebrained or hare-brained [similar to scatter brain]

So, generally, heads, eyes, arms, legs, feet, toes and fingers, and the brain (less often ears and knees) can have ed added to them to form adjectives. There is also lily-livered meaning cowardly. livered comes from liver. And various items with brain.

[I am sure I probably missed some. But that's the general idea.]

Regarding the alternative between 'three-room' and 'three-roomed', Google Ngram Viewer shows strong differences between American and British usage. 'one-room' to 'five-room' has been far more common in AmEng since about 1900, while 'one-roomed' etc is occasionally used. 'one-roomed' etc remained more common in BrEng until about 1950, the two forms were equally used until about 1970, 'one-room' is now more common but 'one-roomed' is still used about 25-50% as often as 'one-room' (compared with about 5% as often in AmEng).

"three-roomed house is wrong" - Not in British English, and not even in American English. Very rare, but not wrong.

As in the other answers, you can indeed say a three roomed house. Either three roomed or three room sound perfectly natural to me (a native English, Australian dialect speaker).

I'd like to offer a few words on what the two different forms tend to imply to me. I emphasize these connotations are extremely subtle, and, as in other answers, any real connotations are probably dying out, with the "right" form increasingly simply defined by customary usage or preference.

Let's take two phrases:

three bedroomed house as against three bedroom house

and

one eyed man as againts one eye man.

The last, "one eye man" is not incorrect, but it would be extremely seldom usage. I don't even believe I've ever heard it.

For me, one would tend to say, "she's building a three bedroomed house" instead of "three bedroom house", but "three bedroom house" is likely more common in real estate advertisements. I believe the former emphasizes the house as a "growing" or "built" entity: it evokes something evolving towards the described form or state. Hence it is used with parts of living animals: they grew in a womb or an egg and came out like that. On the other hand, "three bedroom house" simply, barely denotes a subclass of object with a certain attribute: you don't care that the house was at one stage built into its present form if you're renting it, you simply want to know whether it will fit your needs.

On the other hand, the connotations of "one eye man" are very subtle to describe for me. The best I can do is that I can imagine it might describe a member of an alien race in a scifi movie - a humanoid being whose species' phenotypes only ever have one eye as opposed to a human being. Likewise, I feel it would be more correct to say a "one eye cyclops" as opposed to "one eyed cyclops": cyclopses by definition only ever have one eye, so you would only say the first to poetically emphasize the one eyedness thay they are known for. "One eyed cyclops" would imply that there might be other, many eyed cyclopses walking around.

If you know a bit of German, the effect is, for me, not unlike the distinction between past participles paired with sein as opposed to werden to make a passive, although it is subtler and less "living language" than the German usage.