Just because it once stuck in memory as a result of an by now rather "outdated" American study: It is by no means the case that city dwellers automatically have a better ecological footprint – without wanting to spark a discussion about the details here, newer studies tend to reach different conclusions or at least the status "unclear." This is of course a overall consideration and not only related to the housing situation (energetically, a new single-family house may of course perform worse compared to a new(!) rental apartment), but the overall situation is the more important one for the environment. In short: According to current knowledge, even someone with an ecological mindset does not have to feel bad living in a single-family house. ;)