How house numbers are read
Add the digits of the door number and reduce to one digit. Letters count too: in the Pythagorean table A=1, B=2, C=3 and so on, so 12B becomes 1+2+2 = 5. For flats, most practitioners use the flat number, not the building number; some add both.
Culture shapes real buildings
Number beliefs are not just personal. Many Western high-rises skip a 13th floor label; many buildings in China, Hong Kong and other parts of East Asia avoid floors ending in 4, because "four" sounds like "death" in several Chinese dialects; and addresses or flats with 8 can carry a premium in markets where 8 signals prosperity. If you are buying, those preferences can affect resale demand even if you don't share them.
FAQ
Can I "fix" a house number?
Some practitioners suggest adding a small letter or symbol inside the door to change the total. Official addresses can't be changed this way, so keep your legal address unchanged on documents.