Costessey is a civil parish situated 4 miles (6.4 km) west of Norwich in Norfolk, England. ~Wikipedia
The old building is still there and has undergone some alterations. The modern post office is behind the camera, just on the other side of the mini roundabout.
Kelly’s 1900 Directory of Norfolk lists John Laws as the postmaster around the time of the postcard.
An example of the Costessey postmark from another postcard is shown below.