add validation to checkout page and customer address input fields magento 2

Logged in user checkout Logged out user checkout Logged in user profile page Billing address checkout Shipping address checkout   ALL should have the same input fields to ensure data integrity. For some reason I cannot get this message across to the other developer, this has caused me great embarrassment with the client (again). So below I will list the fields, most are in place but have some errors which I only found yesterday.   BILLING ADDRESS FORM AT CHECKOUT (user logged in OR out) FIRST NAME (LETTERS ONLY, REQUIRED) LAST NAME (LETTERS ONLY, REQUIRED) HOUSE NAME (LETTERS ONLY, NOT REQUIRED) HOUSE NUMBER (NUMBER ONLY, NOT REQUIRED STREET NAME (LETTERS ONLY, REQUIRED) CITY (LETTERS ONLY, REQUIRED) COUNTY (LETTERS ONLY, NOT REQUIRED) POSTCODE (LETTERS AND NUMBERS, UK FORMAT. REQUIRED) COUNTRY (LETTERS ONLY, NOT REQUIRED, DEFAULT UK) PHONE NUMBER (NUMBERS ONLY, REQUIRED)   SHIPPING ADDRESS FORM AT CHECKOUT (user logged in OR out) FIRST NAME (LETTERS ONLY, REQUIRED) LAST NAME (LETTERS ONLY, REQUIRED) HOUSE NAME (LETTERS ONLY, NOT REQUIRED) HOUSE NUMBER (NUMBER ONLY, NOT REQUIRED STREET NAME (LETTERS ONLY, REQUIRED) CITY (LETTERS ONLY, REQUIRED) COUNTY (LETTERS ONLY, NOT REQUIRED) POSTCODE (LETTERS AND NUMBERS, UK FORMAT. REQUIRED) COUNTRY (LETTERS ONLY, NOT REQUIRED, DEFAULT UK) PHONE NUMBER (NUMBERS ONLY, REQUIRED)   USER PROFILE AREA FIRST NAME (LETTERS ONLY, REQUIRED) LAST NAME (LETTERS ONLY, REQUIRED) HOUSE NAME (LETTERS ONLY, NOT REQUIRED) HOUSE NUMBER (NUMBER ONLY, NOT REQUIRED STREET NAME (LETTERS ONLY, REQUIRED) CITY (LETTERS ONLY, REQUIRED) COUNTY (LETTERS ONLY, NOT REQUIRED) POSTCODE (LETTERS AND NUMBERS, UK FORMAT. REQUIRED) COUNTRY (LETTERS ONLY, NOT REQUIRED, DEFAULT UK) PHONE NUMBER (NUMBERS ONLY, REQUIRED)   As you can see from the above input fields, all forms are the same