Adding a County input field into Address in Craft Commerce Any ideas how to add custom fields to the address Model? 

I want to add a free text input for County, so that it can be used for the equivalent in any country, as the shop sells to more than UK. 

Upon doing so, I get the following error when trying to edit an address;

Craft\Commerce_AddressModel and its behaviors do not have a method or closure named "county".
Currently (Dec '16) - custom fields are not able to be used with addresses in Commerce.

Your best best is probably to add a field to your orders and pretend it's on addresses, but obviously if you deal with multiple addresses, as you probably do, that gets a bit tricky quickly.  You could implement a plugin that has it's own table of records that associate addressIDs with a county field.

I am pretty sure it's on the Commerce radar but you're a bit stuck right now I think, without some extra work.