Artificial things are, by definition, unnatural in some sense.
But artifical things clearly exist in the world, and are produced by beings that exist in the world. Hence there is some sense in which things can exist in and as part of the world, yet not be natural.
What's the relevant sense, and where are the boundaries to be found? The most pithy treatment I know of in the D&D context is found in Gygax's AD&D books, particularly the discussin of True Neutral alignment:
The "true" neutral looks upon all other alignments as facets of the system of things. Thus, each aspect - evil and good, chaos and law - of things must be retained in balance to maintain the status quo; for things as they are cannot be improved upon except temporarily, and even then but superficially. Nature will prevail and keep things as they were meant to be, provided the "wheel" surrounding the hub of nature does not become unbalanced due to the work of unnatural forces - such as human and other intelligent creatures interfering with what is meant to be. (PHB p 33)
Absolute, or true, neutral creatures view everything which exists as an integral, necessary port or function of the entire cosmos. Each thing exists as a part of the whole, one as a check or balance to the other, with life necessary for death, happiness for suffering, good for evil, order far chaos, and vice versa. Nothing must ever become predominant or out of balance. Within this noturalistic ethos, humankind serves a role also, just as all other creatures do. They may be more or less important, but the neutral does not concern himself or herself with these considerations except where it is positively determined that the balance is threatened. (DMG p 33)
Nature is "the cosmos" that is in a state of balance as a result of the interaction of its constituent elements and processes. Intelligent beings are a risk to that balance, as they bring their own goals and purposes which are not necessarily integrated into the balance of natural elements and processes. It's easy to see how this idea relates to certain real world religious and philosophical positions (eg Stoicism; some forms of Taoism and Taoist-influenced Buddhism; some strands of contemporary environmentalism). And it helps us see the difference between natural and unnatural human activity - the latter consists in purposive activity undertaken with indifference to its impact upon the balance of natural elements and processes. Building a small homestead or even village probably doesn't count; raising an army and mining the ore and then forging the arms and armour to equip them almost certainly does!
How exactly this fits into our understanding of "cold" iron and faeries I'll leave for others to work out.
A car isn't required for any sort of natural process and doesn't come from any sort of natural process. The same goes for houses and many other acts that humans engage in.
