Changing address?? How do I add letters onto a house number?

I Currently live in a newly converted flat. Im upstairs %26amp; someone is downstairs, but address is stil just 38.We need to change this to 38a and 38b...How do I go about doing this.Changing address?? How do I add letters onto a house number?
The owner of the flat(not the renters)..must go to your local mail provider..royal mail in uk...usps in united states and apply for the new address...Changing address?? How do I add letters onto a house number?
The builder will have registered the site with the local council. Also they would pass this on to Royal Mail. I suggest ringing the council first and they can tell you.
just decide between yourselves and leave a note for the post person.