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.空房 would be pronounced kong1fang2, right?
It is interesting that we picked the tone subconsciously. Although we've been following some guidelines, we never noticed that. Even being asked, after thinking, few people can tell the guideline.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).
That's surprising.few people can tell the guideline.