Prince William and Kate Middleton's royal home is referred to as an 'apartment', but it's not like a unit like we're thinking.

Many think of an apartment as a two-bedroom place, but Kate and William's apartment is more like a mini-mansion on the grounds of Kensington Palace.

The residence is known as Apartment 1A and is where the Duke and Duchess of Cambridge live with Prince George, Princess Charlotte and Prince Louis when they're in London.

A royal author has revealed what it's like in the apartment.

"It's enormous... and it's like a piece of countryside in London," revealed royal author Ingrid Seward to True Royalty's Royal Beat.

"It has 20 rooms from the basement to the attic, it is not a small house," she explained.

"All of these royal residences at Kensington Palace are called apartments, which of course makes people immediately think they are flats like the American term for an apartment. They are not."

The homes are "joined terrace houses", with a lift, a gym and a $310,000 kitchen that the couple paid to renovate.

Duchess Kate has helped decorate the home with the aid of an interior designer, though the furnishes have likely changed over the years as the family has grown.