Best way to validate street address and keep in database?

Developing website for a client. Visitors of website will be able to register their businesses. And I need to get address of their business and somehow validate it and save it.

There are no limits. It can be for example by putting "street 123" into field, me calling some API and offering him "city1 street 123" or "city2 street 123". Or visitor placing a pin on to map.

Client would also like to filter businesses by distance (eg. no more then 20 miles from me). And client of course doesn't want to pay anything for this verification service (there would be no more than ~100 registrations per day).

Anybody has an experience what is the best way to approach this? Do you have some API or service that you using for validating addresses?

 

What country is it for? Some national postal operators might have an API or an address database that you could get a license for to validate addresses against

 

What country is it for and how good does the validation have to be?

If you need really strict validation you could find a service with an API that sends out letters with codes to customers addresses. The customer can then enter this code into your system when they receive it at their address. This is common with banks in some countries.

 

Once you asked about my country I realized, that we used to have one local website that was offering map for our country (same as Google Maps but limited to our country). So I checked them out, they are still in business and they are offering free unlimited API free for commercial use.... . Which does exactly what I'm looking for.
Thanks!