The Buddhist doctrine on impermanence pertains only to the conditioned world which is the world of the appearance of things as contrasted with true reality which is unborn, unoriginated, etc. As we would expect of any conditioned thing such as our corporeal body or all conditioned things, there is nothing unconditioned to be had since the nature of phenomena or conditioned things is to arise and eventually pass away which is not the case with the unconditioned (A. i. 152).
Comprehending the scope of the relativity of conditioned existence means that we can find no unchanging self-nature (svabhāva), or intrinsic nature, in the conditioned—not even a Buddha-nature. If we did, this nature would be marked with arising which would eventually pass away. It could no longer be valid as unconditioned.
Fundamentally, our conditioned world originates in dependence upon the constructing mind which, itself, is self-existent and unconditioned but which, at the same time, can condition itself, making conditioned things arise which are never other than niḥsvabhāva (destitute of svabhāva) being illusory constructs.
If we are looking for self-nature (svabhāva) it is not to be found in each individual thing or things (all of which are conditioned) but only in that which is, so to speak, the constructor of the conditioned (which is never other than unconditioned). In Zen we know this by various names such as 'unconditioned Mind', 'pure Mind', 'unborn Mind' etc.
“Just as a painter mixes and blends the various color [in his paintings], so by delusory projections of Mind are made the various forms of all phenomena . . . . The Buddha differs not from the Mind, nor sentient beings from the Buddhas, yet, both Mind and Buddha are by nature infinite! He who knows the Mind as the creator of all worlds, sees the Buddha and his true essence” (Mahāvaipulya Buddhāvataṃsaka Sūtra).
For the average person this is difficult to get their mind around. The entire world we live in is not real although it seems otherwise. Still, this illusory world arises from the real, which we do not know, and returns to it moment by moment. Buddhism wants us to personally intuit the true source of all (the conditioned) which is unconditioned.