The regulations of this article shall apply to existing lots, structures, uses and signs that do not conform to the regulations of the zoning district in which they are located and were either in existence prior to the effective date of this chapter, or subsequent amendments, or are rendered nonconforming by the adoption of this chapter. As such, they shall be known and regarded as nonconforming, and the following regulations, as applicable, shall apply to them. Provided, however, that the burden of proof shall remain upon the landowner to prove that a lot, structure, use or sign is lawfully nonconforming.