Almost every position in a numerology chart is derived from a person: from the name given at birth, from the date of birth, or from the name currently in use. The Address Number is the exception. It is derived from the building, and it is held to describe the character of life lived there rather than the character of whoever lives there. Two unrelated households at the same address are given the same reading.
The calculation takes the street number and nothing else. The digits are added and reduced to a single figure, and a letter appearing in the number, as in 221B, is converted through the same letter table the name positions use, so that 221B gives two plus two plus one plus two. The street name, the town and the postcode are excluded. Master numbers are kept unreduced where they appear, on the same rule as elsewhere in the system.
Practitioners use it as an environmental reading rather than a personal one. A dwelling reducing to four is described as suited to steady work and unsuited to a household wanting movement; one reducing to five is described the other way round. The advice that follows is normally about fit: whether a particular household should take a particular place, and what the place will tend to draw out of them.
Because it attaches to a building rather than to a person, it is the one position in the chart that changes when you move and changes for nobody else.