This autumn, Prince William and Kate Middleton are planning to move in permanently in Kensington Palace. And by doing so, the Duke and Duchess of Cambridge are now building a two-storey extension that will allow their family to fit in as the house is getting an overhaul.

According to Mail Online, the 18th-century Orangery that is built to protect Queen Anne's citrus trees is now used as a restaurant for visitors. The 165-foot long and two-storey basement extension can provide an extra space for more than 1,500 square meters.

Reports claimed that the basement will house a ceremonial dress collection and further provide offices for over 100 staff. Prince William and Duchess Kate Middleton will be moving on this fall to apartment 1A at Kensington Palace in London.

According to some reports, the Royal family will start moving in when Prince William finishes his two-year job being a pilot with the East Anglian Air Ambulance. When the Royal family moves in, the present residents of the Kensington Palace will have to make several spaces. Prince Harry also lives nearby and he is in Nottingham Cottage.

The Historic Royal Palaces have enforced for a big extension in the palace, an 'iceberg' basement, a two-storey underground expansion. According to reports, there are more than £4.5 million of taxpayers' has been spent over the royal couple's apartment in the Palace which has two kitchens and twenty-two rooms.

Business Insider has reported that a huge extension in Kensington Palace is a rare one, so the Chelsea council and the Palace will hold momentarily the request. Though reports already suggest that a two-storey basement might not be allowed due to some disruption, there will likely be exceptions for the royals.

Meanwhile, the application has already been forwarded and a decision from the council is set to be at the end of April. Duke and Duchess of Cambridge will expect the council to say yes probably for George and Charlotte extra space to play.

Prince William and Duchess Kate application state that the addition of the basement storey is needed to allow the accommodation of administration. It will necessarily be moved out the rooms from the Royal Household in Kensington Palace.