Look up field to give a result

Design help General
1

I would like to add search field on our website that ask for a postcode and then gives a result based on the input. In this case telling customers if they live in a hard water area. I guess I would need some sort of lookup table. Could somebody please point me in the correct direction,

Here is my site Read-Only: LINK
(how to share your site Read-Only link)

2

Hi Paul,

Roughly how many zip codes are there?
Does the list change often?
Does your client need to be able to change the list themselves?