To determine if an address exists or not official sources like bpost (https://www.bpost.be/nl/adresvalidatie-tool) and Geopunt (https://www.geopunt.be/) should be used. Another source that could be used (especially for Wallonia where Geopunt is not available) is CadGIS (https://eservices.minfin.fgov.be/ecad-web/#/).
In Address Verification a query address can be rated Valid if it exists and is unambiguous and real (see Guidelines section 3), and Invalid if it does not exist or does not refer to one clear location. In the case the query address is Invalid it can be rated reasonable or not reasonable. See the examples below for more details.
Query Address: meir 78 2000 Antwerpen
The query address is a full and unambiguous address which can be verified by both bpost and Geopunt as it appears as a suggestion (see below) so it should be rated Valid.
Query Address: Nieuwstraat 158 8000 Brugge
The above Query Address is not in bpost or Geopunt (see below). In the case the Query Address does not exist it should be rated Invalid.
(8) KLEINE NIEUWSTRAAT – 8000 BRUGGE
In the case an address is rated Invalid it can be reasonable or unreasonable. For an address to be rated as reasonable, it must refer to a location that exists in the real world, and must at least fulfil one of the conditions listed in the Guidelines section 4 (page 15). See examples below.
Query Address: Nieuwstraat 158 8000 Brugge
First of all, for an address to be rated as reasonable, it must refer to a location that exists in the real world. The address above (which is the same address as the previous example) does not exist and does not refer to a location that exists in the real world so it is unreasonable.
Query Address: 3500 Hasselt
The above query Address is a locality and does refer to a location that exists in the real world; But this locality has a large area (see figure below) and does not fulfil one of the conditions listed in section 4, page 15 of the Guidelines. This address is also unreasonable.
Query Address: Beemdstraat 9000 Gent
In this example the Query Address is for an entire street; however, this street is very small and is within eyesight (which is one of the possible conditions a reasonable address should fulfil, see GL page 15), see figure below.
Because the Query Address refers to a location that exists in the real world and fulfils one of the possible conditions on page 15 (within eyesight) it is reasonable.