Postcodes are the sole responsibility of Royal Mail and they are only assigned by Royal Mail following receipt of address confirmation from the Street Naming and Numbering (SN&N) team. The developer should contact
Royal Mail to receive the allocated postcode on 08456 011 110, option 3 and then option 1.
You can also contact Royal Mail using the above link or phone number if you are a resident who is unsure of your postcode or experiencing any problems with it.