空房 would be pronounced kong1fang2, right?
This is an interesting question. I hear people say both kong1fang2 and kong4fang2, and I think each has a point. For both 空kong1 and 空kong4 have the meaning of vacant/vacancy, though the former is adjective and the latter is a noun (as explained in dictionary). However, in these words I see 空kong4 can also be an adjective: 空地 空座 空场(this might be dialectic) where I would consider kong1 as incorrect, or at least unnatural. In the case of 空车, however, kong1 is definitely the natural choice. 空房 is a little in between, and either would do IMO. Actually 空房 is not a very common expression in Chinese, I think 空房间 sounds better, where 空 should be pronounced as kong1, or 空余房间,where 空 would be kong4.
Edit: I just noticed Super XW's answer above. Actually it might have something to do with the tone of the following/ending character. 空车,空屋,空窗,etc, all end with tone 1, and 空 is only acceptable as kong1. In words that end with 3 or 4 (空地,空座,空位,空当)空 should be pronounced as kong4. As for words ending with tone 2... well like 空房 or 空床 I think both are pronounced but maybe kong4 is somewhat preferred.
This rule doesn't apply to 空白/空缺 as here 空 is a noun (not adjective+noun but noun+noun structure), thus kong4.
Edit again (please disregard the above)- No, I was deceived. I just thought about 空碗 空瓶 空盒 空井 etc. and had to agree with Super XW. We use kong1 when referring to the physical status of a container, and kong4 when expressing that something is not occupied/used/claimed. Therefore 空房 can be pronounced either way depending on what you want to stress- that the rooms (房间 is better here) or suites are empty (kong1) or that nobody is living or using them (kong4).