They built it from bits of TARDISes in the Bubble universe's junkyard, a task which Idris called "impossible". The TARDISes had arrived through the Bubble universe rift and been consumed by House and their Time Lord occupants killed. This TARDIS consisted solely of a console (complete with time rotor) and only three walls. Because it lacked a proper shell, the Doctor said it would be very dangerous to fly.