Also called the Royal Pavilion, the Prince Regent had the same man who designed Carlton House renovate (enlarge) the structure. What did the existing building look like? Who knows, Brighton was a crumbling fishing town only then starting to rebuild its image as a seaside retreat for the rich and famous. The area wasn't important until George took an interest in it (to take in the waters) and spent a fortune turning it into a gorgeous architectural structure.
Don't believe me? Here's the wiki excerpt with comments in bold.
In 1787 the designer of Carlton House, Henry Holland, was employed to enlarge the existing building, which became one wing (only the one) of the Marine Pavilion, flanking a central rotunda, which contained only three main rooms, a breakfast room, dining room and library, (I like how the paragraph reads contained only 3 main rooms, a breakfast room, dining room, & library. I mean come on! He has a library what more is there?) fitted out in Holland's French-influenced neoclassical style, with decorative paintings by Biagio Rebecca. In 1801-02 the Pavilion was enlarged with a new dining room and conservatory, to designs of Peter Frederick Robinson, in Holland's office. The Prince also purchased land surrounding the property, on which a grand riding school and stables were built in an Indian
style in 1803-08, to designs by William Porden that dwarfed the Marine Pavilion,
in providing stabling for sixty horses.
To be fair, the place is bloody gorgeous! And admission isn't so expensive that spending a few hours there to enjoy the excellent design and beautiful gardens seems worth it. If I ever get to England, this is on my list of places to see.
First 2 pictures are from the wiki article: Royal Pavilion at night and "The richly decorated Banqueting Room at the Royal Pavilion, from John Nash's Views of the Royal Pavilion (1826)"; the 3rd picture is from : Brighton Royal Pavilion and Museums Home