It's bigger…and more spartan…on the inside. For the 50th anniversary special, the traveling police box is geting a makeover.
Source: ign.com
Everything old is new again. When The Snowmen airs this Christmas, the Doctor will be traveling in a more sterile TARDIS. The interior of the police box has changed over the years, but since the relaunch in 2005 it has maintained an organic appearance.
The new design appears to be taken from concept art drawn in 1989 (picture below) before the show was cancelled.
Source: harnois75.deviantart.com