The Tank is to open at 10am on Saturday, (April 23) on the former McDonald’s site.
Measuring just 102 square foot, it beats the previous record of 105, according to the mastermind behind it Daniel Slade.
He said: “Because it is a temporary structure the records office won’t issue a certificate.
“I did it because I wanted to attract attention, so sadly it will be going, but it will appear at special occasions which we’ll publish on our website and on Facebook.”
Mr Slade took on the lease of the prominent High Street unit, which had been empty since November 2014, just before Christmas.
The full plans are still under wraps, but he admitted the sizable site will get a transformation which has been 12 years in the making.
Mr Slade said: “I have been saving very hard to bring something to Bognor, something it deserves.
“The coffee bar is just an idea I thought up one night and then built with my bare hands over three weeks before getting the professionals in to do the electrics and things like that.”
Despite its size the bright blue bar boasts a few seats and wall-mounted aquariums home to live fish.
It will sell fresh coffee and ‘the best hot chocolates in town’.
Town centre manager Toyubur Rahman is one of many who are excited for Mr Slade’s bigger plans to be revealed.
He said: “I think it will be a marvellous addition to the town. It is something we need because we have a lack of nighttime and evening economy.
“It will raise the bar of what we do, he is a local person who is passionate about the town centre and wants to see the town doing well.”