The Istana Nurul Iman palace is the official residence of the Sultan of Brunei, Hassanal Bolkiah. The palace is located on a leafy, riverside sprawl of hills on the banks of the Brunei River directly south of Brunei's capital, Bandar Seri Begawan, no more than a few miles outside the city center. Its name is taken from Arabic, meaning Palace of the Light of Faith. It is the largest residential palace in the world and the world's largest residence of any type. It was designed by Leandro V. Locsin, and built by the Filipino firm, Ayala International.
The palace is used for all State functions. It is both the seat of Brunei's government and the location of the prime minister's office. In addition to Audience and State Rooms, there is a Throne Chamber used for various occasions such as the proclamation of the Crown Prince and the annual Birthday Investiture.
The palace is not open to the public except on the annual Islamic celebration of Hari Raya Aidilfitri (the festival at the end of the Muslim fasting month) when the palace receives about 110,000 visitors over a three-day period where they receive compliments such as food for visiting as well as green packets containing money for young children. The palace is also open to Muslims during the Ramadhan period for 10 days for the Bertedarus prayer gathering.
Istana Nurul Iman is undisputedly the world's largest residential palace currently in use for state functions.
However, it is primarily a residence while in many European palaces, most of the actual space in the palace is used for administrative and state purposes as is the case with Buckingham Palace. Thus, at 828,818 square feet (77,000 m2), Buckingham Palace bears the title of the world's largest "working" royal palace.
Still, Buckingham Palace is not even the largest palace in Europe. At 1,453,122 square feet (134,999.5 m2), the Royal Palace of Madrid, in Spain, is larger. However, the king of Spain resides elsewhere and it is used only for ceremonial purposes and heads of state visits thus it loses the title "largest working palace" to Buckingham Palace. In addition, some claim that Stockholm Palace, the 608 room official, but unoccupied, headquarters of the Swedish monarchy, is "the largest royal palace in the world that is still in use for its original purpose." Such a claim obviously ignores that the larger Istana Nurul Iman was built for the purpose that it currently serves today, as a royal palace.
Further complicating the title to "world's largest palace" is The Forbidden City which is the world's largest palace complex, but it is neither occupied as a residence nor is it one continuous building. In fact, it contains 800 different buildings. The total square footage of the complex is just less than 7,750,000 square feet (720,000 m2), but with the vast open courtyards of the complex, the totaled square footage of the Forbidden City's collective buildings is likely smaller than that of Istana Nurul Iman.
And none of the above addresses "castles." Windsor Castle lays claim to the title "world's largest inhabited castle" while the Prague Castle castle lays claim to the outright title of "world's largest castle." Windsor Castle has a floor area of just over 484,000 square feet while the Prague Castle is much like the Forbidden City in that it is merely a complex containing many separate buildings, some of which are palaces.
Istana Nurul Iman: 2,152,782 square feet (200,000.0 m2)
The Royal Palace of Madrid: 1,453,122 square feet (134,999.5 m2) [1]
Buckingham Palace: 828,818 square feet (77,000 m2) [2]
The Palace of Versailles: 551,218 square feet (51,210 m2) [3]
Windsor Castle: 484,374 square feet (45,000 m2) [4]
Rashtrapati Bhavan,India: 200,000 square feet (19,000 m2)