As PhilD says, the average ATZ rises to 2000' from the surface.
I'm always surprised by the number of pilots who choose to overfly at 2000' (or thereabouts) on the QNH. If the airfield elevation is, say 600', flying at 2000' on the QNH means that the aircraft is 1400' aal and, in this example, might conflict with aircraft descending on the deadside from a standard overhead join. Using the same example, the aircraft would have to fly at 2600' on the QNH to be flying at the upper limits of the ATZ.