Which deficiency would be classified as external obsolescence?

External obsolescence is an type of depreciation; a defect, usually incurable, caused by negative influences outside a site and generally incurable on the part of the owner, landlord, or tenant. Also known as a loss of value from all causes outside the property itself.

When a residential building is situated in a commercial environment

Residential and commercial buildings dont tend mix well in suburban areas. It's usually a negative factor when houses are located next to restaurants, retail, gas stations, offices. You could classify this as a deficiency